AN1376 APPLICATION NOTE 25W QUASI-RESONANT FLYBACK CONVERTER FOR SET-TOP BOX APPLICATION USING THE L6565 This document describes a reference design of a 25W Switch Mode Power Supply dedicated to Set-Top Box application. The board accepts full range input voltage (90 to 265Vrms) and delivers 5 outputs. It is based on the new controller L6565, working in variable frequency mode. 1 INTRODUCTION Set-Top Boxes are growing very fast and they are becoming very popular in all Countries either for satellite or cable decoding. Hence the market is asking for solutions having high cost effectiveness, providing for good performances, low noise, small volumes at low cost. The Quasi-resonant operation and the high flexibility of the L6565 make it a very suitable device, able to satisfy all the requirements with only few external components. The board has been designed with mixed technology components, both PTH and SMT. For this reason some components are doubled, in accordance with their ratings. AN1376/0904 Rev. 2 1/35 AN1376 APPLICATION NOTE 2 MAIN CHARACTERISTICS The main characteristics of the SMPS are listed here below: ■ INPUT VOLTAGE: Vin: 90 - 264 Vrms f: 45-66 Hz ■ OUTPUT VOLTAGES: Vout (V): Iout (A): Pout (W): STABILITY NOTES 3.3 2.00 6.6 +/- 2% 5 1.1 5.5 +/- 2% (A) 12 0.7 8.4 +/- 5% (B) 7 0.5 3.5 +/- 8% (C) 30 0.015 0.45 +/- 2% (D) POUT (W) = 24.45 NOTES: (A) Dedicated to 5V digital circuitry and to 3.3v local post regulators (B) Dedicated to SCART, LNBP21 for satellite STB. For other applications the current is 0.4A (C) Dedicated to 5V local post regulators (D) Dedicated to tuner STAND-BY No stand-by mode is foreseen by equipment ■ OVERCURRENT PROTECTION On all outputs, with auto-restart at short protection ■ PCB TYPE & SIZE: Cu Single Side 35 um, FR-4, 122.5 x 75 mm ■ SAFETY: In acc. with EN60950, creepage and clearance minimum distance 6.4mm ■ EMI: In acc. with EN50022 Class B ■ 2/35 F1 C32 2N2 - 0805 C23 1N0- 0805 R28 24K - 0805 R11 4K7 - 0805 R2 NTC_16R S236 COMP FB VFF Z CD C22 220PF- 0805 2 1 3 8 VCC C13 1N0- 1KV GND I SEN OUT 6 4 7 4 D2 2W08G- GS C21 0805 - NOT MOUNTED R22 0R0 - 0805 R3 33R- 0805 Q1 STP4NK60ZFP TO-220 HEAT SINK ABL LS220 R4 100K - 0805 C12 C16 100N 1N0- 1KV I C1 L6565 - DI P8 5 4 1 2 3 FASTON 6mm 3 2 L1 39mH - EPCOS 1 J P1A FASTON 6mm FUSE 2A C9 100N J P1 R1 270K- 1206 D5 LL4148 C11 22uF- 25V YXF D4 LL4148 C10 47uF- 400V C33 100N - 0805 R6 0R47 - 1/2W PTH R27 120K - 0805 C17 220PF-1KV HRR P8 J UMPER R15 R14 470K- 1206 270K- 1206 R12 470K- 1206 GBI Q2 BC856 D9 SMCJ 130CA 7 C3 2200uF- 16V YXF C2 2 4 3 I C2 TL431ACD - SO- 8 L2 10u ELC06D VOUT I C3 LD1086V50 C25 100N - 0805 C28 100N - 0805 C20 22uF-50V YXF C7 100uF-16V - YXF Q3 BC847B R10 220R - 0805 R9 82R - 0805 Q4 BC847B R25 1K8 - 0806 D12 LL4148 C24 100N - 0805 C26 100N - 0805 C27 100N - 0805 R26 1K0 - 0806 C4 100uF- 16V - YXF L4 2u7 ELC08D R18 47R - 1/ 2W PTH R21 470R - 1206 L3 10u ELC06D GND C29 C5 100N - 0805 TO-220 HEAT SINK 100uF-16V - YXF ABL LS220 VI N C6 100uF-16V - YXF R19 1K0 - 1/2W PTH R23 NOT MOUNTED C14 330N - 1206 R16 470R - 1/2W PTH D10 BZV55- C15 C18 22uF- 50V YXF D11 BZV55- C15 C15 NOT MOUNTED R8 560R- 0805 R7 2K7 - 0805 YXF 9 2200uF- 16V 2200uF- 16V YXF C1 D8 STPS10L60FP TO-220 HEAT SI NK 8 10 11 D7 STPS10L60FP R17 1R0 - 2W - PTH C19 2200uF-16V - YXF D6 STPS2H100U R20 10K - 1206 C30 220PF- 1KV HRR R24 180R 1/ 2W PTH D1 SMBYT01- 400 12 13 1 OPT1 SFH617A- 4 R5 3K3 - 0805 C31 220P - 0805 R13 NOT MOUNTED 6 4 STTH1L06U D3 P TH D9A NOT MOUNTED 2 14 T1 2414. 0011 r ev. C1 1 8 J P2 MKS 1858- 6- 0- 808 3 . 3 V @2 A GND GND 5 V @1 . 1 A 7 V @0 . 5 A GND 3 0 V @0 . 0 1 5 A 1 2 V @0 . 7 A 3 C8 2N2- 2KV ( Y1) AN1376 APPLICATION NOTE ELECTRICAL DIAGRAM Figure 1. Electrical Diagram 3/35 AN1376 APPLICATION NOTE The switching frequency (minimum is ~30 kHz @Vin = 80 VDC) has been chosen to get a compromise between the transformer size and the harmonics of the switching frequency, in order to optimise the input filter size and its cost. The MOSFET is a standard and cheap 600V-1.76Ω typ., TO-220FP. It needs a small heat sink. The transformer reflected voltage is 90V, providing enough room for the leakage inductance voltage spike with still margin for reliability. The network D9+D3 clamps the peak of the leakage inductance voltage spike. These two components are SMT, allowing cost saving of the manual labour with respect to a passive solution, needing manual insertion on the PCB. A 220pF HV capacitor has been added across the drain to optimise MOSFET losses by a small snubbing effect on the drain voltage rate of rise. The controller L6565 is activated by a couple of dropping resistors (R1+R14, for voltage and power rating reasons) that draws current from the DC bus and charges the capacitor C11. This circuit dissipates only about 240mW @ 264 Vac, thanks to the extremely low start-up current. During the normal operation the controller is powered by the transformer via the diode D4. The network Q101, C102, R104 acts as a spike killer, improving the auxiliary voltage fluctuations and the performance in short circuit. R12+R15 and R11 compensate for the power capability change vs. the input voltage (Voltage Feed-forward). A 1nF ceramic capacitor bypasses any noise on pin #3 to ground (C23). The current flowing in the transformer primary is sensed by the resistor R6. The circuit connected to pin1 (FB) provides for the over voltage protection in case of feedback network failures and open loop operation. The output rectifiers have been chosen in accordance with the maximum reverse voltage and power dissipation. The rectifiers for 3.3V and 7V outputs are Schottky, type STPS10L60FP. These diodes are low forward voltage drop, hence dissipating less power with respect to standard types. Both are the same to decrease the component diversity, as well as for the capacitors C1 to C3 and C19. The diode D8 needs a small heat sink, as indicated on the BOM. The other two output rectifiers are SMT, fast recovery. The snubber R102 and C101 damps the oscillation produced by the diode D1 at MOSFET turn-on. The output voltage regulation is performed by secondary feedback on the 3.3V output, while for other voltages the regulation is achieved by the transformer coupling. The feedback network is the classical TL431 driving an optocoupler, in this case an SFH617A-4, insuring the required insulation between primary and secondary. The opto-transistor drives directly the COMP pin of the controller. The 5V output is linearly post-regulated from the 7V output to get a very stable voltage. A zener regulator assures the 30V stability at low cost. The 5V regulator needs to be dissipated. A small LC filter has been added on the +12V, +7V, +3.3V in order to filter the high frequency ripple without increasing the output capacitors. A 100nF capacitor has been connected on each output, very close to the output connector soldering points to limit the spike amplitude. The input EMI filter is a classical Pi-filter, 1-cell for differential and common mode noise. A NTC limits the inrush current produced by the capacitor charging at plug-in. The transformer is slot type, manufactured by Eldor Corporation, in accordance with the EN60950. Here following some waveforms during the normal operation at full load: 4/35 AN1376 APPLICATION NOTE Figure 2. Vds & Id @ Full Load Vin = 115 Vrms - 50 Hz Vin = 220 Vrms - 50 Hz CH1: DRAIN VOLTAGE; CH2: RAIN CURRENT - VR.SENSE (R6) The pictures above show the drain voltage and current at the nominal input mains voltage during normal operation at full load. The Envelope acquisition of the scope provides for the possibility to see the modulation of the two waveforms due to the input voltage ripple. Figure 3. Vds & Id @ Full Load (Vin = 265 Vrms - 50 Hz) This picture gives the measurement of the drain peak voltage at full load and maximum input mains voltage. The voltage peak, which is 548V, assures a reliable operation of the PowerMOS with a good margin against the maximum BVDSS. CH1: DRAIN VOLTAGE; CH2: RAIN CURRENT - VR.SENSE (R6) Figure 4. Vin = 265 Vrms - 50 Hz, @FULL LOAD: DIODE PIV CH3: +35V DIODE: ANODE VOLTAGE; CH4: +12V DIODE: ANODE VOLTAGE CH3: +7V DIODE: ANODE VOLTAGE; CH4: +3V3 DIODE: ANODE VOLTAGE 5/35 AN1376 APPLICATION NOTE The maximum PIV of the diodes has been measured during the worst operating condition and it is indicated on the right of each picture. The margin, with respect to the maximum voltage sustained by the diodes, assures a safe operating condition for the devices. Here following the most salient controller IC signals are depicted. In both the pictures is possible to distinguish clean waveforms free of hard spikes or noise that could affect the controller correct operation. Figure 5. Vin = 115 Vrms - 50 Hz CH1: 4 Vin = 220 Vrms - 50 Hz VPIN5 - ZCD CH2: VPIN4 - ISENSE CH3: VPIN7 - OUT CH4: VPIN2 - COMP CROSS REGULATION In the following tables the output voltage cross regulation is measured with static and dynamic loads and the overall efficiency of the converter measured at different input voltages. All the output voltages have been measured after the load connector soldering point of the STB motherboard. The length of the connection cable is 100 mm. ■ FULL LOAD Vout [V} = 30.06 7.23 12.297 3.278 4.492 Vin [Vrms]= 115 Iout [A] = 0.015 0.500 0.702 2.073 1.103 Iin [Arms] = 0.51 Pout [W] = 0.451 3.615 8.632 6.795 4.955 Pin [W] = VUNREG = 37.2 VC11 = 11.88 fS = 41÷51 kHz 6/35 PoutTOT [W] = 24.448 EFF. = 67.91% ALL VOLTAGES ARE WITHIN TOLERANCE 36.0 AN1376 APPLICATION NOTE Vout [V} = 30.5 7.197 12.19 3.279 4.94 Vin [Vrms]= 220 Iout [A] = 0.015 0.5 0.702 2.073 1.103 Iin [Arms] = 0.31 Pout [W] = 0.458 3.599 8.557 6.797 5.449 Pin [W] = VUNREG = 37.2 VC11 = 12.04 35.1 PoutTOT [W] = 24.860 EFF. = 70.82% fS = 66÷68 kHz ALL VOLTAGES ARE WITHIN TOLERANCE The efficiency of the converter is not very high but it is heavily affected by 5V the linear regulator delivering 1.1 A. Delivering 1.6A on the 7V output but removing the 5V regulator the efficiency measured is 75.6% @220Vac and 76.9% at 115Vac. ■ Reduced Load - for Cable STB, without the LNB Vout [V} = 31.4 7.18 12.11 3.359 4.965 Vin [Vrms]= 115 Iout [A] = 0 0.25 0.3 1.008 0.55 Iin [Arms] = 0.26 0.000 1.795 3.633 3.386 2.731 Pin [W] = Pout [W] = VUNREG = 35.5 VC11 = 11.39 16.8 PoutTOT [W] = 11.545 EFF. = 68.72% fS = 83÷89 kHz ALL VOLTAGES ARE WITHIN TOLERANCE Vout [V} = 31.4 7.16 12.08 3.36 4.975 Vin [Vrms]= 220 Iout [A] = 0 0.25 0.3 1.008 0.55 Iin [Arms] = 0.17 0.000 1.790 3.624 3.387 2.736 Pin [W] = Pout [W] = VUNREG = 35.4 VC11 = 11.5 fS = 112 kHz 17.6 PoutTOT [W] = 11.537 EFF. = 65.55% ALL VOLTAGES ARE WITHIN TOLERANCE The above tables shown the output voltage measured applying the same loads that we could have in case of a different Set-top Box type is powered (e.g. a terrestrial or cable) without the LNB block of the satellite antenna. Like before all the output voltages are within the tolerances. 7/35 AN1376 APPLICATION NOTE ■ Reduced Load - 9W Vout [V} = 31.6 7.16 12.77 3.36 4.96 Vin [Vrms]= 115 Iout [A] = 0 0.300 0.051 1.008 0.6 Iin [Arms] = 0.22 0.000 2.148 0.651 3.387 2.976 Pin [W] = PoutTOT [W] = 9.162 Pout [W] = VUNREG = 36 VC11 = 11.48 14.2 EFF. = 64.52% fS = 94÷101 kHz ALL VOLTAGES ARE WITHIN TOLERANCE Vout [V} = 31.5 7.16 12.76 3.36 4.96 Vin [Vrms]= 220 Iout [A] = 0 0.300 0.051 1.008 0.602 Iin [Arms] = 0.15 Pout [W] = 0.000 2.148 0.651 3.387 2.986 Pin [W] = VUNREG = VC11 = 35.8 15.2 PoutTOT [W] = 9.172 11.7 EFF. = 60.34% fS = PFM/PWM ALL VOLTAGES ARE WITHIN TOLERANCE Even still reducing the load till 9W, Thanks to the good coupling of the transformer, all the output voltages are still in tolerance. ■ At No-Load (Output connector unplug) Vout [V] = Vout [V] = 8/35 30.4 30.4 7.20 7.20 12.15 12.0 3.39 3.38 5.00 5.00 Vin [Vrms]= 220 Pin [W] = 1.6 Vin [Vrms]= 115 Pin [W] = 1.5 AN1376 APPLICATION NOTE Figure 6. Vin = 115 Vrms - 50 Hz Unplugging the output connector the circuit is still able to maintain all the voltages perfectly under control and within the tolerance. Hence a perfect functionality of the circuit is achieved also in this abnormal condition. During the no load operation the circuit works in burst mode and, thanks to the controller functionality, the switching frequency inside the Burst pulses is kept at low. This provides for a low power consumption of the power supply, making it suitable to support stand-by operation with low consumption from the mains. It has to be kept into account that this circuit has not been optimized for the Stand-by operation, hence it could be improved. Vin = 220 Vrms - 50 Hz CH1: VPIN5 - ZCD CH2: VPIN4 - ISENSE CH3: VPIN7 - OUT CH4: VPIN2 - COMP 9/35 AN1376 APPLICATION NOTE 5 OUTPUT VOLTAGE RIPPLE @FULL LOAD In the following picture all the output voltage ripple at switching and mains frequency are measured. As per the previous measures, the probes have been connected on test points after the output flat cable. As shown in the pictures, the ripple and the spikes are very low. Figure 7. @115 VAC - 50Hz @220 VAC - 50Hz 10/35 CH2: +7 Vout CH2: +30 Vout CH3: +3.3 Vout CH3: +5 Vout CH4: +12 Vout AN1376 APPLICATION NOTE Figure 8. @115 VAC - 50Hz - LINE FREQUENCY RIPPLE CH1: VC10 CH2: +30 Vout CH2: +7 Vout CH3: +5 Vout CH3: +3.3 Vout CH4: +12 Vout The low frequency residual ripple compared with the ripple across C10 (input Elcap) shows an excellent rejection of the circuit (>80 dB). 6 MEASUREMENT OF THE RMS CAPACITOR CURRENTS The tables show the rms currents flowing in the output capacitors at 115Vac and 220Vac, full load. All the rms currents are within the rating of the capacitor type indicated (Rubycon, YXF series). This avoids the component overstress that should affect the reliability and/or the expected lifetime of the SMPS . @ 115Vac: ICAP C1 = 1.02 ARMS @ 115Vac: ICAP C3 = 1.78 ARMS @ 115Vac: ICAP C19 = 1.15 ARMS @ 115Vac: ICAP C18 = 140 mARMS @ 220Vac: ICAP C1 = 0.7 ARMS @ 220Vac: ICAP C1 = 1.4 ARMS @ 220Vac: ICAP C19 = 0.92 ARMS @ 220Vac: ICAP C18 = 130 mARMS 11/35 AN1376 APPLICATION NOTE 7 DYNAMIC LOAD TESTS Regulated Output Load condition: +5V, +7V, +12V, +30V: FULL LOAD +3,3V: LOAD 50 %÷100%, 70Hz Figure 9. @115 VAC - 50Hz @220 VAC - 50Hz CH3: +3V3 Vout at test points CH1: +30 Vout CH4: +3V3 Iout CH2: +12 Vout +3V3 Vout before L4 CH3: +7 Vout CH4: +3V3 Iout R1: The pictures show the output voltage regulation against a dynamic load variation of the feed backed voltage, at the nominal input voltage values. As shown in the left pictures the response after the connector is not very good from the peak point of view, even if the response is quite fast. Making the same measure before the filter inductor (L4), at the feed back divider connection points, the response is much better (≈2.2 %). This means that the filter inductor heavily 12/35 AN1376 APPLICATION NOTE affect the response. To avoid any expensive solution to improve it the better way is to measure the voltage regulation during the normal operation, powering the real load circuitry. This, because there are some local capacitors or filters helping a lot the regulation. Moreover, normally the dynamic load changes are less than the testing value indicated. The regulation for all the other output voltage is good, remaining well within tolerances. ■ Unregulated Outputs The following tests show the response of the output voltages varying the load for each unregulated output. The load conditions are specified at the right of each picture. The regulation has been tested at both the nominal mains voltages. Figure 10. @115 VAC - 50Hz CH1: +12 Vout CH1: +12 Vout CH2: +7 Vout CH2: +7 Vout CH3: +3V3 Vout CH3: +3V3 Vout CH4: +12V Iout CH4: +5V Iout +3,3V +5V, +7V, +30V: FULL LOAD +12V: DYNAMIC LOAD 0.1 to 0.7A, 70Hz +3,3V +5V, +7V, +30V: FULL LOAD +5V: DYNAMIC LOAD 0.5 to 1.1A, 70Hz @ 220Vac the waveforms have the same amplitude. 5V modulation: 20 mVpp @ 220Vac the waveforms have the same amplitude. ALL THE VOLTAGES ARE WITHIN TOLERANCES, AT BOTH INPUT MAINS VOLTAGES 13/35 AN1376 APPLICATION NOTE Figure 11. @115 VAC - 50Hz +3,3V +5V, +7V, +30V: FULL LOAD +7V: DYNAMIC LOAD 0.1 to 0.5A, 70Hz 5V modulation: 20 mVpp @ 220Vac the waveforms have the same amplitude. ALL THE VOLTAGES ARE WITHIN TOLERANCES, AT BOTH INPUT MAINS VOLTAGES 8 CH1: +12 Vout CH3: +3V3 Vout CH2: +7 Vout CH4: +7V Iout START-UP BEHAVIOUR @FULL LOAD Figure 12. @115 VAC - 50Hz @220 VAC - 50Hz @85 VAC - 50Hz CH1: CH2: 14/35 +12 Vout +5 Vout @265 VAC - 50Hz CH3: CH4: +3.3 Vout +7V Iout AN1376 APPLICATION NOTE In the previous 4 pictures there are the rising slopes at full load of the more significant output voltages at nominal, minimum and maximum input mains voltage. As shown in the pictures, the rising times are constant and there is only a slight difference for the 5V rise time, with respect to the other outputs. This characteristic is quite important when the loads are a µP and its peripherals as in our case, to avoid problem at start-up. At minimum voltage a super imposed ripple at line frequency is present, due to the high ripple at the input that is not completely rejected by the loop before reaching the steady state operation. This because while the input voltage is rising, the ripple valley voltage is less than the minimum operating voltage of the circuit, therefore the ripple it is properly rejected only when it reaches that value. 9 WAKE-UP TIME In the following pictures there are the waveforms with the wake-up time measures at the nominal input mains. Obviously, due to the circuitry characteristics, the wake-up time is not constant but it is dependent on the input voltage. The measured time at 115 and 220 Vac are 1.2 and 0.6 second, which are rather common values for this kind of Power Supplies. The worst condition, of course, is at 85 Vac when the start-up time becomes around 1.7 seconds, which is quite a long time even if still acceptable. This because there is anyway the startup time of the STB which is longer. Additionally, the 85Vac input mains is a steady state voltage but it is not a very common value. Figure 13. @115 VAC - 50Hz @220 VAC - 50Hz In Figure 14 there are the waveforms at minimum and maximum voltage with a magnification of the time base: on the picture is clearly indicated that no any overshoot, undershoot, dip or lost of control happens during the power supply start-up phase. Obviously also the nominal voltages are been detected without showing any abnormal behaviour. 15/35 AN1376 APPLICATION NOTE Figure 14. @85 VAC - 50Hz @265 VAC - 50Hz CH1: VDD CH1: VDD CH2: VC11 (Vaux) CH2: VC11 (Vaux) CH3: +3V3 Vout CH3: +3V3 Vout 10 TURN-OFF Even at turn off the transition is clean, without any abnormal behaviour like restart or glitches both on the primary or secondary side. Figure 15. @85 VAC - 50Hz 16/35 @265 VAC - 50Hz CH1: VDD CH1: VDD CH2: VC11 (Vaux) CH2: VC11 (Vaux) CH3: +3V3 Vout CH3: +3V3 Vout AN1376 APPLICATION NOTE 11 SHORT-CIRCUIT TESTS @ FULL LOAD The short circuit tests have been done in two phases, both making the test shorting by a power switch the output electrolytic capacitor or making the short by the active load option. This gives an idea about the circuit behaviour with a hard short (at very low impedance) or with a "soft" short that could happen on the STB main board, having slightly higher impedance. All the tests have been done at maximum and minimum input voltage. For all conditions the drain voltage is always below the BVDSS, while the mean value of the output current has a value close to the nominal one, then preventing component melting for excessive dissipation. The auto-restart is correct at short removal in all conditions. Figure 16. 7V OUTPUT: SHORT C3 @85 VAC @265 VAC CH1: DRAIN VOLTAGE CH1: DRAIN VOLTAGE CH2: VC11 (Vaux) CH2: VC11 (Vaux) CH4: ISHORT CIRCUIT CH4: ISHORT CIRCUIT As clearly indicated by the waveforms, the circuit start to work in hic-cup mode, so maintaing the mean value of the current at levels supported by the component rating. Because the working time and the dead time are imposed by the charging and discharging time of the auxiliary capacitor C11, it is proportional to the input mains voltage. Figure 17. 7V OUTPUT: SHORT BY ACTIVE LOAD @85 VAC @265 VAC CH1: DRAIN VOLTAGE CH1: DRAIN VOLTAGE CH2: VC11 (Vaux) CH2: VC11 (Vaux) CH4: ISHORT CIRCUIT CH4: ISHORT CIRCUIT 17/35 AN1376 APPLICATION NOTE As expected the circuit protects itself as well. The secondary peak current is obviously lower, due to the higher circuit impedance. Figure 18. 3V3 OUTPUT: SHORT C1 @85 VAC @265 VAC CH1: DRAIN VOLTAGE CH1: DRAIN VOLTAGE CH2: VC11 (Vaux) CH2: VC11 (Vaux) CH4: ISHORT CIRCUIT CH4: ISHORT CIRCUIT Like the previous output voltage the controller keeps under control the circuit preventing in all conditions the circuit from catastrophic failures. This happens even shorting the output by the active load. Figure 19. 12V OUTPUT: SHORT C19 @85 VAC @265 VAC CH1: DRAIN VOLTAGE CH1: DRAIN VOLTAGE CH2: VC11 (Vaux) CH2: VC11 (Vaux) CH4: ISHORT CIRCUIT CH4: ISHORT CIRCUIT Even the 12V output is well protected against shorts, either by a power switch or by the active load 18/35 AN1376 APPLICATION NOTE Figure 20. 35V OUTPUT: SHORT C18 @85 VAC @265 VAC CH1: DRAIN VOLTAGE CH1: DRAIN VOLTAGE CH2: VC11 (Vaux) CH2: VC11 (Vaux) CH4: ISHORT CIRCUIT CH4: ISHORT CIRCUIT The above pictures are relevant to a hard short by switch of the output capacitor C11. The short by active load has not been tested because the load is not connected on this point, but after the zener limiting resistors. The short circuit on the +30V has not been tested because the power rating of the limiting resistors in series to the zener diode is not enough to insure a reliable protection against longterm short circuits. A solution could be to PUT a PTC resistor or similar component, or changethe present resistor with a fusible resistor. Figure 21. 5V OUTPUT: SHORT BY ACTIVE LOAD @115 VAC A short circuit made on the flat cable soldering points with a power switch provides for the current limiting intervention of the regulator at 2A (LD1086V50) Then, due to the internal over temperature protection the regulator starts to switch on and off itself, always keeping the output current under control. Hence, with this transformer and this regulator, an overcurrent on the 5V is not able to provide for the hic-cup working mode the previous tests but anyway all the most important circuit parameters are below any dangerous overstress point. CH1: DRAIN VOLTAGE CH2: VC11 (Vaux) CH4: ISHORT CIRCUIT 19/35 AN1376 APPLICATION NOTE 12 SHORT CIRCUIT PROTECTION @ LOW LOAD After the full load tests some checks on the short circuit protection with reduced loads have been done. @Half Load 35V 12V 7V 5V 3.3V 15mA 0.25 A 0.35A 0.55A 1A PoutTOT = 12.6W At Vin = 115Vac: shorting each output by the active load the over current protection works correctly, providing for the hic-cup working mode, except for the 5V which is protected by the current limiting of the linear regulator. At Vin = 220Vac: the circuit behaves like at 115V. @Reduced Load - 1 35V 12V 7V 5V 3.3V 15mA 0.5 A 0A 0A 1A PoutTOT = 9.5W At Vin = 115Vac: shorting the 3.3V, 7V, 12V and 35V it provides for the hic-cup working mode of the circuit. At Vin = 220Vac: the behaviour is the same. @Reduced Load - 2 35V 12V 7V 5V 3.3V 15mA 0.4 A 0A 0A 0A Both at 115V and 220V the circuit is still protected against short circuits on all the outputs 13 SHORT CIRCUIT PROTECTION @ NO LOAD Even in this abnormal condition, with the output connector unplugged, a short on the outputs provides for the same results of the previous tests, both at 115Vac or at 220 Vac. Figure 22. 3.3V OUTPUT: SHORT @NO LOAD @115 VAC 20/35 @220 VAC CH1: DRAIN VOLTAGE CH1: DRAIN VOLTAGE CH2: VC11 (Vaux) CH2: VC11 (Vaux) CH3: 3.3V OUTPUT CH4: 3.3V OUTPUT AN1376 APPLICATION NOTE 14 SHORT CIRCUIT OF THE OUTPUT RECTIFIERS A frequent problem in a power supply is relevant to the protection of the SMPS itself: thus sometimes it is easy to find circuits with a good protection capability against shorts of the load but which are not able to survive in case of a very hard short like an output electrolytic capacitor or a diode. Besides, in case of a rectifier shorted the equivalent circuit changes and the energy is delivered even during the on time, like in forward mode. To insure reliable operation of the design, even this fault condition has been simulated for each rectifier. Thanks to the controller functionality, the SMPS can withstand this failure, working in burst mode as visible in the pictures, Figure 23. RECTIFIERS SHORT: @FULL LOAD - 220 VAC 3.3V 7V CH1: DRAIN VOLTAGE CH1: DRAIN VOLTAGE CH2: VPIN4 - ISENSE CH2: VPIN4 - ISENSE 35V 12V CH1: DRAIN VOLTAGE CH1: DRAIN VOLTAGE CH2: VPIN4 - ISENSE CH2: VPIN4 - ISENSE In case of an output diode short, the current sensing voltage exceeds a second protection level, then the controller stops the operation, so avoiding the destruction of the components at primary side. The controller remains in off-state until the voltage across the Vcc pin decreases below the UVLO threshold. Then it try to restart and it will switch off again until the secondary short is removed. This provides for the hic-cup working mode, preventing the circuit destruction. The operating frequency inside the burst is the internal timer one (~2.5 Khz). 21/35 AN1376 APPLICATION NOTE 15 SWITCH ON AND TURN OFF IN SHORT CIRCUIT CONDITION ■ ■ FULL LOAD SHORT ON 3V3 BY ACTIVE LOAD The following pictures describe the SMPS behaviour during the start-up phase with an output voltage shorted. As clearly visible the circuit starts correctly then it works in hic-cup mode protecting itself. The start-up phase is clean in all conditions, without showing any dangerous transition for the SMPS circuitry. Figure 24. START @85 VAC - 50Hz @265 VAC - 50Hz Figure 25. TURN-OFF @85 VAC - 50Hz @265 VAC - 50Hz CH1: VDD CH1: VDD CH2: VC11 (Vaux) CH2: VC11 (Vaux) CH3: +3V3 Vout CH3: +3V3 Vout Even at turn off in short circuit the SMPS functionalities are good, protecting properly the circuit. No any abnormal transition or level has been observed during the tests. 22/35 AN1376 APPLICATION NOTE 16 OVER VOLTAGE PROTECTION A dangerous fault that could happen in case is the failure of the feedback circuitry. If this occurs, the SMPS output voltages can get high values, depending on the load by each output and the transformer coupling between the windings. Consequently, the rectifiers and the output capacitors are overstressed and can be destroyed. To avoid the SMPS failure a suitable protection circuit has been added. Then the circuit has been tested opening the loop, giving the following results: 3.3V OUTPUT: @ full load 3.3V OUTPUT: @ No load @115V – 50Hz @220V – 50Hz @115V – 50Hz @220V – 50Hz V3V3: 4.02 V V3V3: 4.08 V V3V3: 4.64 V V3V3: 4.67 V 23/35 AN1376 APPLICATION NOTE 17 CONDUCTED NOISE MEASUREMENTS (Pre-Compliance Test) The following pictures are the peak and quasi-peak conducted noise measurements at full load and nominal mains voltages. The limits shown on the diagrams are the EN55022 CLASS B ones, which is the most widely rule for domestic equipments like a STB. As visible on the diagrams there is a good margin of the measures with respect to the limits, either in peak or quasi-peak mode. The detail of the filtering components used is on the right of each diagram. Figure 26. Vin = 115 Vrms 50 Hz – FULL LOAD Limits: EN55022 CLASS B PEAK MEASURE BOARD #2 C9 = 100nF EPCOS L = 39 mH EPCOS C16 = 100nF EPCOS TRAFO 2412.0011 REV. C1 Pout = 25W QUASI-PEAK MEASURE BOARD #2 C9 = 100nF EPCOS L = 39 mH EPCOS C16 = 100nF EPCOS TRAFO 2412.0011 REV. C1 Pout = 25W 24/35 AN1376 APPLICATION NOTE Figure 27. Vin = 220 Vrms 50 Hz – FULL LOAD Limits: EN55022 CLASS B PEAK MEASURE BOARD #2 C9 = 100nF EPCOS L = 39 mH EPCOS C16 = 100nF EPCOS TRAFO 2412.0011 REV. C1 Pout = 25W QUASI-PEAK MEASURE BOARD #2 C9 = 100nF EPCOS L = 39 mH EPCOS C16 = 100nF EPCOS TRAFO 2412.0011 REV. C1 Pout = 25W 25/35 AN1376 APPLICATION NOTE 18 THERMAL MEASURES In order to check the reliability of the design a thermal mapping by means of an IR Camera was done. Here below the thermal measures on the board, at both nominal input voltages at ambient temperature (24 °C) are shown. The pointers A¸E have been placed across some key components, affecting the reliability of the circuit. The points correspond to the following components: component side A Input coil - L1 B PowerMOS – Q1 C +7V diode – D7 D +3.3V diode – D8 E +5V regulator – IC3 As shown on the maps, all the other points of the board are within the temperature limits assuring a reliable performance of the devices. Figure 28. @115VAC - FULL LOAD COMPONENT SIDE A B C D E 47.41°C 47.24°C 78.39°C 59.02°C 70.67°C SMD SIDE 26/35 AN1376 APPLICATION NOTE The highest temperatures are for the NTC thermistor, the filter inductor, the input bridge, the clamp diode (D9), the 5V regulator and the output diodes D7 and D6. The temperature rise of the transformer is around 40 °C. Regarding the thermistor, the bridge and the output diodes the temperature rise is compatible with reliable operation of the circuit. Figure 29. @220VAC - FULL LOAD COMPONENT SIDE A B C D E 39.31°C 54.21°C 80.15°C 60.05°C 72.00°C SMD SIDE At 220Vac the input circuitry is thermally less stressed and generally the component temperature rise is lower. 27/35 AN1376 APPLICATION NOTE 19 CONCLUSIONS A SMPS for Set-Top Box has been completely designed, assembled and tested, giving positive results from all the different aspects (Component Stress, Functionalities, Protections, EMI, thermal behaviour). The design meets also the low-cost requirement, a key driver in the Consumer Electronic market. 20 REFERENCES [1] "L6561-based Fly-back Converters" (AN1060) [2] "L6565 Quasi-Resonant Controller " (AN1326) [3] "How to handle Short Circuit Conditions with ST's Advanced PWM Controllers" (AN1215) 21 ANNEX 1 Table 1. PART LIST Designator Part Type Description Supplier 1 C1 2200uF-16V YXF ELCAP RUBYCON 2 C10 47uF-400V ELCAP SAMHWA 3 C11 22uF-25V YXF ELCAP RUBYCON 4 C12 1N0-1KV 30LVD10 CERCAP HV CERA-MITE 5 C13 1N0-1KV 30LVD10 CERCAP HV CERA-MITE 6 C14 330N - 1206 CHIP CAPACITOR AVX 7 C15 0805 - NOT MOUNTED CHIP CAPACITOR AVX 8 C16 100N-275Vac - B81133 X CAP EPCOS 9 C17 220PF-1KV HRR CERCAP HV MURATA 10 C18 22uF-50V YXF ELCAP RUBYCON 11 C19 2200uF-16V - YXF ELCAP RUBYCON 12 C2 2200uF-16V YXF ELCAP RUBYCON 13 C20 22uF-50V YXF ELCAP RUBYCON 14 C21 0805 - NOT MOUNTED CHIP CAPACITOR AVX 15 C22 220PF-0805 CHIP CAPACITOR AVX 16 C23 1N0-0805 CHIP CAPACITOR AVX 17 C24 100N-0805 CHIP CAPACITOR AVX 18 C25 100N-0805 CHIP CAPACITOR AVX 19 C26 100N-0805 CHIP CAPACITOR AVX 20 C27 100N-0805 CHIP CAPACITOR AVX 21 C28 100N-0805 CHIP CAPACITOR AVX 22 C29 100N-0805 CHIP CAPACITOR AVX 23 C3 2200uF-16V YXF ELCAP RUBYCON 24 C30 220PF-1KV HRR CERCAP HV MURATA 25 C31 220P - 0806 CHIP CAPACITOR AVX 26 C32 2N2-0805 CHIP CAPACITOR AVX 27 C33 100N-0805 CHIP CAPACITOR AVX 28 C4 100uF-16V - YXF ELCAP RUBYCON 29 C5 100uF-16V - YXF ELCAP RUBYCON 30 C6 100uF-16V - YXF ELCAP RUBYCON 28/35 AN1376 APPLICATION NOTE Table 1. PART LIST (continued) Designator Part Type Description Supplier 31 C7 100uF-16V - YXF ELCAP 32 C8 2N2-4KV (Y1) 44LD22 CERCAP-SAFETY RUBYCON CERA-MITE 33 C9 100N-275Vac - B81133 X CAP EPCOS 34 D1 SMBYT01-400 RECTIFIER STMICROELECTRONICS 35 D10 BZV55-C15 ZENER DIODE PHILIPS SEMICOND. 36 D11 BZV55-C15 ZENER DIODE PHILIPS SEMICOND. 37 D12 LL4148 GEN. PURPOSE DIODE PHILIPS SEMICOND. 38 D2 2W08G-GS BRIDGE RECTIFIER GEN. SEMICOND. 39 D3 STTH1L06U RECTIFIER STMICROELECTRONICS 40 D4 LL4148 GEN. PURPOSE DIODE PHILIPS SEMICOND. 41 D5 LL4148 GEN. PURPOSE DIODE PHILIPS SEMICOND. 42 D6 STPS2H100U RECTIFIER STMICROELECTRONICS 43 D7 STPS10L60FP RECTIFIER STMICROELECTRONICS 44 D8 STPS10L60FP RECTIFIER STMICROELECTRONICS SMCJ130CA (GBI) - SMC TRANSIL STMICROELECTRONICS 1,5KE150A - NOT MOUNTED TRANSIL STMICROELECTRONICS 45 D9 46 D9A 47 F1 48 HS1 FUSE 2A ABL LS220 HEAT SINK FOR Q1 ABL WICKMANN 49 HS2 ABL LS220 HEAT SINK FOR IC3 ABL 50 HS3 6073 HEAT SINK FOR D8 THERMALLOY 51 IC1 L6565 - DIP8 INTEGRATED CIRCUIT STMICROELECTRONICS 52 IC2 TL431ACD INTEGRATED CIRCUIT STMICROELECTRONICS 53 IC3 LD1086V50 LIN. REGULATOR STMICROELECTRONICS 54 JP1 FASTON 6mm CONNECTOR 55 JP1A FASTON 6mm CONNECTOR 56 JP2 MKS1858-6-0-808 CONNECTOR - 8 POLES STOCKO 57 L1 B82732-R2701-B30 2*39 mH - FILTER COIL EPCOS 58 L2 10u ELC06D INDUCTOR PANASONIC 59 L3 10u ELC06D INDUCTOR PANASONIC 60 L4 2u7 ELC06D INDUCTOR PANASONIC 61 OPT1 SFH617A-4 OPTOCOUPLER INFINEON 62 P1 0R0-1206 CHIP RESISTOR BEYSCHLAG 63 P2 0R0-1206 CHIP RESISTOR BEYSCHLAG 64 P3 0R0-1206 CHIP RESISTOR BEYSCHLAG 65 P4 0R0-1206 CHIP RESISTOR BEYSCHLAG 66 P5 67 P6 68 69 70 PCB 71 Q1 72 Q2 BC856 SMALL SIGNAL BJT ZETEX 73 Q3 BC847B SMALL SIGNAL BJT STMICROELECTRONICS JUMPER, WIRE 0R0-1206 CHIP RESISTOR BEYSCHLAG P7 0R0-1206 CHIP RESISTOR BEYSCHLAG L5 NOT MOUNTED - SHORTED JUMPER, WIRE STP4NK60ZFP POWER MOSFET 35u, SINGLE SIDE, FR4 STMICROELECTRONICS 29/35 AN1376 APPLICATION NOTE Table 1. PART LIST (continued) Designator Part Type Description Supplier 74 Q4 BC847B SMALL SIGNAL BJT 75 R1 270K-1206 CHIP RESISTOR STMICROELECTRONICS BEYSCHLAG 76 R10 220R - 0805 CHIP RESISTOR BEYSCHLAG 77 R11 4K7 - 0805 CHIP RESISTOR BEYSCHLAG 78 R12 470K-1206 CHIP RESISTOR BEYSCHLAG 79 R13 0805 - NOT MOUNTED CHIP RESISTOR BEYSCHLAG 80 R14 270K-1206 CHIP RESISTOR BEYSCHLAG 81 R15 470K-1206 CHIP RESISTOR BEYSCHLAG 82 R16 470R - 1/2W PTH FUSE RESISTOR PTH NEOHM 83 R17 1R0 - 2W PTH POWER RESISTOR NEOHM 84 R18 47R - 1/2W PTH SFR RESISTOR PTH BEYSCHLAG 85 R19 1K0 - 1/2W PTH SFR RESISTOR PTH BEYSCHLAG 86 R2 NTC_16R S236 NTC THERMISTOR EPCOS 87 R20 10K - 1206 CHIP RESISTOR BEYSCHLAG 88 R21 470R - 1206 CHIP RESISTOR BEYSCHLAG 89 R22 0R0-0805 CHIP RESISTOR BEYSCHLAG 90 R23 0805 - NOT MOUNTED CHIP RESISTOR BEYSCHLAG 91 R24 180R 1/2W PTH SFR RESISTOR PTH BEYSCHLAG 92 R25 1K8 - 0805 CHIP RESISTOR BEYSCHLAG 93 R26 1K0 - 0805 CHIP RESISTOR BEYSCHLAG 94 R27 120K - 0805 CHIP RESISTOR BEYSCHLAG 95 R28 24K - 0805 CHIP RESISTOR BEYSCHLAG 96 R3 33R-0805 CHIP RESISTOR BEYSCHLAG 97 R4 100K - 0805 CHIP RESISTOR BEYSCHLAG 98 R5 3K3 - 0805 CHIP RESISTOR BEYSCHLAG 99 R6 0R47 - 1/2W PTH SFR RESISTOR PTH BEYSCHLAG 100 R7 2K7 - 0805 CHIP RESISTOR BEYSCHLAG 101 R8 560R-0805 CHIP RESISTOR BEYSCHLAG 102 R9 82R - 0805 CHIP RESISTOR BEYSCHLAG 103 T1 2414.0011 rev. C1 TRANSFORMER ELDOR CORPORATION 30/35 AN1376 APPLICATION NOTE 22 ANNEX 2 Figure 30. SILK SCREEN -TOP SIDE Figure 31. SILK SCREEN -BOTTOM SIDE 31/35 AN1376 APPLICATION NOTE Figure 32. COPPER TRACKS 32/35 AN1376 APPLICATION NOTE Table of Contents 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 INTRODUCTION................................................................................................................................1 MAIN CHARACTERISTICS ...............................................................................................................2 ELECTRICAL DIAGRAM ...................................................................................................................3 CROSS REGULATION ......................................................................................................................6 OUTPUT VOLTAGE RIPPLE @FULL LOAD...................................................................................10 MEASUREMENT OF THE RMS CAPACITOR CURRENTS ...........................................................11 DYNAMIC LOAD TESTS .................................................................................................................12 START-UP BEHAVIOUR @FULL LOAD .........................................................................................14 WAKE-UP TIME ...............................................................................................................................15 TURN-OFF .......................................................................................................................................16 SHORT-CIRCUIT TESTS @ FULL LOAD .......................................................................................17 SHORT CIRCUIT PROTECTION @ LOW LOAD ............................................................................20 SHORT CIRCUIT PROTECTION @ NO LOAD...............................................................................20 SHORT CIRCUIT OF THE OUTPUT RECTIFIERS .........................................................................21 SWITCH ON AND TURN OFF IN SHORT CIRCUIT CONDITION ..................................................22 OVER VOLTAGE PROTECTION.....................................................................................................23 CONDUCTED NOISE MEASUREMENTS (PRE-COMPLIANCE TEST)........................................24 THERMAL MEASURES ...................................................................................................................26 CONCLUSIONS ...............................................................................................................................28 REFERENCES.................................................................................................................................28 ANNEX 1 .........................................................................................................................................28 ANNEX 2 ..........................................................................................................................................31 33/35 AN1376 APPLICATION NOTE Table 2. Revision History Date Revision November 2001 1 First Issue September 2004 2 Changed the style look & feel. Changed the Figure 1. Changed in "Table 1. Part List" the items 39, 43, 44, 70, 73 & 74. 34/35 Description of Changes AN1376 APPLICATION NOTE The present note which is for guidance only, aims at providing customers with information regarding their products in order for them to save time. As a result, STMicroelectronics shall not be held liable for any direct, indirect or consequential damages with respect to any claims arising from the content of such a note and/or the use made by customers of the information contained herein in connection with their products. Information furnished is believed to be accurate and reliable. However, STMicroelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of STMicroelectronics. Specifications mentioned in this publication are subject to change without notice. This publication supersedes and replaces all information previously supplied. STMicroelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of STMicroelectronics. The ST logo is a registered trademark of STMicroelectronics. All other names are the property of their respective owners © 2004 STMicroelectronics - All rights reserved STMicroelectronics group of companies Australia - Belgium - Brazil - Canada - China - Czech Republic - Finland - France - Germany - Hong Kong - India - Israel - Italy - Japan Malaysia - Malta - Morocco - Singapore - Spain - Sweden - Switzerland - United Kingdom - United States of America www.st.com 35/35