Application Information STR3A100 Series PWM Off-Line Switching Regulator ICs General Description STR3A100 series are power ICs for switching power supplies, incorporating a power MOSFET and a current mode PWM controller IC in one package. Including a startup circuit and a standby function in the controller, the product achieves low power consumption, low standby power, and high cost-effectiveness in power supply systems, while reducing external components. Features and Benefits • Current mode PWM control • Built-in Random Switching function: reduces EMI noise, simplifies EMI filters, and cuts cost by external part reduction • Built-in Slope Compensation function: avoids subharmonic oscillation • Built-in Leading Edge Blanking (LEB) function • Auto Standby function: ▫ Input power, PIN < 10 mW at no load with low power consumption shunt regulator ▫ Normal load operation: PWM switching ▫ Light load operation: Standby mode (Burst oscillation) • Soft Start function: reduces stress on internal power MOSFET and secondary output rectifier diode at startup • Protection Functions: ▫ Overcurrent Protection function (OCP); pulse-by-pulse, built-in compensation circuit to minimize OCP point variation on AC input voltage Figure 1. The STR3A100 series package is a fully molded, industrystandard DIP8. ▫ Overload Protection function (OLP); auto restart, built-in timer, reduces heat during overload condition, and no external components required ▫ Overvoltage Protection function (OVP); latched shutdown, or auto restart for D and HD types ▫ Thermal Shutdown function (TSD); latched shutdown, or auto restart for D and HD types Applications Switching power supplies for electronic devices such as: • Stand-by power supply for LCD/PDP television, desktop PC, multi-function printer, audio equipment, and so forth • Small switched-mode power supply (SMPS) for printer, BD/DVD player, set-top box, and so forth • Auxiliary power supply for air conditioner, refrigerator, washer, dishwasher, and so forth The product lineup for the STR3A100 series provides the following options: Part Number STR3A151 STR3A152 STR3A153 STR3A154 STR3A155 fOSC (kHz) 67 POUT* (W) MOSFET OVP/TSD VDSS(min) (V) RDS(on)(max) (Ω) 230 VAC 85 to 265 VAC 650 4.0 3.0 1.9 1.4 1.1 24 30 36 40 43 16 23 30 32 35 Latched 24 30 36 40 43 16 23 30 32 35 Auto restart 26 29 35 17 20 29 Auto restart STR3A151D STR3A152D STR3A153D STR3A154D STR3A155D 67 650 4.0 3.0 1.9 1.4 1.1 STR3A161HD STR3A162HD STR3A163HD 100 700 4.2 3.2 2.2 Part Number Assignment: *The listed output power is based on the thermal ratings, and the peak output power can be 120% to 140% of the value stated here. At low output voltage and short duty cycle, the output power may be less than the value stated here. STR3A100-AN Rev.1.1 SANKEN ELECTRIC CO., LTD. STR3A1 nn a a 1 2 34 1. Product series name 2. ID number for VDSS and RDS(ON) of the incorporated power MOSFET 3. fOSC: 67 kHz, or 100 kHz for H type 4. OVP, TSD protection: latched, or auto restart for D type Functional Block Diagram Control Part VCC 2 D/ST Startup UVLO Reg PWM Oscillator VREG OVP 5, 6, 7, 8 TSD DRV SQ R OCP VCC OLP Feedback control FB/OLP 4 Drain Peak Current compensation LEB Slope compensation S/OCP GND 1 3 Pin List Table Pin-out Diagram S/GND 1 8 D/ST VCC 2 7 D/ST GND 3 6 D/ST FB/OLP 4 5 D/ST Number Name 1 S/OCP Function MOSFET source and input of Overcurrent Protection (OCP) signal 2 VCC Power supply voltage input for Control Part and input of Overvoltage Protection (OVP) signal 3 GND Ground 4 FB/OLP 5, 6,7, 8 D/ST Feedback signal input for constant voltage control signal and input of Overload Protection (OLP) signal MOSFET drain pin and input of the startup current Table of Contents Specifications 3 Package Outline Drawing Package Diagram Absolute Maximum Ratings Electrical Characteristics Typical Application 3 3 4 5 7 Functional Description 8 Startup Operation 8 Startup Period Undervoltage Lockout (UVLO) Circuit Bias Assist Function Auxiliary Winding STR3A100-AN Rev.1.1 8 8 9 9 Soft-Start Function Constant Output Voltage Control Automatic Standby Mode Function Random Switching Function Overcurrent Protection Function (OCP) Overvoltage Protection Function (OVP) Overload Protection Function (OLP) Thermal Shutdown Function (TSD) Design Notes 10 11 12 12 12 13 14 14 15 Peripheral Components 15 Phase Compensation 15 PCB Trace Layout and Component Placement 15 SANKEN ELECTRIC CO., LTD. 2 Package Diagram DIP8 package 9.4 ±0.3 5 1 4 6.5 ±0.2 8 1.0 +0.3 -0.05 +0.3 1.52 -0.05 3.3 ±0.2 7.5 ±0.5 4.2 ±0.3 3.4 ±0.1 (7.6 TYP) 0.2 5 + 0. - 0.01 5 ０～１５° ０～１５° 2.54 TYP 0.89 TYP 0.5 ±0.1 Unit： mm STR3A15x markings 3A15x SK YMD Pb-free. Device composition compliant with the RoHS directive. XXXX STR3A15xD markings 3A15xD SK YMD XXXX STR3A100-AN Rev.1.1 Part Number Lot Number Y is the last digit of the year (0 to 9) M is the month (1 to 9, O, N, or D) D is a period of days: 1 – 1st to 10th 2 – 11th to 20th 3 – 21st to 31st Sanken Control Number STR3A16xHD markings 3A16xH SK YMD Part Number Lot Number Y is the last digit of the year (0 to 9) M is the month (1 to 9, O, N, or D) D is a period of days: 1 – 1st to 10th 2 – 11th to 20th 3 – 21st to 31st Sanken Control Number XXXX SANKEN ELECTRIC CO., LTD. Part Number Lot Number Y is the last digit of the year (0 to 9) M is the month (1 to 9, O, N, or D) D is a period of days: 1 – 1st to 10th 2 – 11th to 20th 3 – 21st to 31st Sanken Control Number 3 Electrical Characteristics • Refer to the datasheet of each product for these details. • The polarity value for current specifies a sink as "+ ," and a source as “−,” referencing the IC. Absolute Maximum Ratings Unless specifically noted, TA is 25°C Characteristic Drain Peak Current Avalanche Energy Symbol IDPEAK EAS Rating Unit STR3A151 STR3A151D STR3A161HD Notes 3.6 A STR3A152 STR3A152D STR3A162HD 4 A STR3A163HD Pins 4.8 A STR3A153 STR3A153D 5.2 A STR3A154 STR3A154D 6.4 A STR3A155 STR3A155D 7.2 A Single pulse 8−1 STR3A151 STR3A151D Single pulse, ILPEAK = 2.13 A 53 mJ STR3A152 STR3A152D Single pulse, ILPEAK = 2.19 A 56 mJ STR3A153 STR3A153D Single pulse, ILPEAK = 2.46 A 72 mJ STR3A154 STR3A154D Single pulse, ILPEAK = 2.66 A 83 mJ STR3A155 STR3A155D Single pulse, ILPEAK = 3.05 A 110 mJ STR3A161HD Single pulse, ILPEAK = 1.43 A 23.8 mJ STR3A162HD Single pulse, ILPEAK = 1.58 A 29 mJ STR3A163HD Single pulse, ILPEAK = 1.88 A 8−1 41 mJ −2 to 6 V 2−3 32 V 4−3 −0.3 to 14 V 4−3 1.0 mA 1.68 W 1.76 W 1.81 W S/OCP Pin Voltage VOCP 1−3 Control Part Input Voltage VCC FB/OLP Pin Voltage VFB FB/OLP Pin Sink Current IFB STR3A151 STR3A151D STR3A152 STR3A152D STR3A161HD STR3A162HD MOSFET Power Dissipation PD1 STR3A153 STR3A153D STR3A154 STR3A154D STR3A163HD Mounted on 15 mm × 15 mm printed circuit board 8−1 STR3A155 STR3A155D Control Part Power Dissipation PD2 2−3 1.3 W Operating Ambient Temperature TOP – −40 to 125 °C Storage Temperature Tstg – −40 to 125 °C Channel Temperature Tch – 150 °C STR3A100-AN Rev.1.1 VCC × ICC SANKEN ELECTRIC CO., LTD. 4 Electrical Characteristics of Control Part Unless specifically noted, TA is 25°C, VCC = 18 V Characteristic Symbol Operation Start Voltage VCC(ON) Operation Stop Voltage* VCC(OFF) Circuit Current in Operation ICC(ON) Minimum Start Voltage VST(ON) Startup Current ISTARTUP Startup Current Threshold Biasing Voltage* VCC(BIAS) Average Operation Frequency fOSC(AVG) Frequency Modulation Deviation Maximum Duty Cycle Leading Edge Blanking Time OCP Compensation Coefficient OCP Compensation Duty Cycle Limit Test Conditions VCC = 12 V Pins Min. Typ. Max. Unit 2–3 13.8 15.3 16.8 V 2–3 7.3 8.1 8.9 V 2–3 − − 2.5 mA 8–3 − 40 − V 2–3 −3.9 −2.5 −1.1 mA 2–3 8.5 9.5 10.5 V STR3A15x STR3A15xD 8–3 60 67 74 kHz STR3A16xHD 8–3 90 100 110 kHz Δf STR3A15x STR3A15xD 8–3 − 5 − kHz STR3A16xHD 8–3 − 8 − kHz DMAX STR3A15x STR3A15xD 8–3 65 74 83 % STR3A16xHD tBW DPC VCC = 13.5 V 8–3 77 83 89 % STR3A15x STR3A15xD – − 350 − ns STR3A16xHD – − 280 − ns STR3A15x STR3A15xD – − 17 − mV/μs STR3A16xHD – − 27 − mV/μs DDPC − − 36 − % OCP Threshold Voltage at Zero Duty Cycle VOCP(L) 1–3 0.69 0.78 0.87 V OCP Threshold Voltage at 36% Duty Cycle VOCP(H) 1–3 0.79 0.88 0.97 V Maximum Feedback Current IFB(MAX) 4–3 −110 −70 −35 μA Minimum Feedback Current IFB(MIN) 4–3 −30 −15 −7 μA FB/OLP Oscillation Stop Threshold Voltage VFB(OFF) STR3A151 STR3A151D STR3A152 STR3A152D STR3A153 STR3A153D STR3A16xHD VCC = 32 V 4–3 1.09 1.21 1.33 V STR3A154 STR3A154D STR3A155 STR3A155D VCC = 32 V 4–3 0.85 0.98 1.09 V OLP Threshold Voltage VFB(OLP) VCC = 32 V 4–3 7.3 8.1 8.9 V OLP Operation Current ICC(OLP) VCC = 12 V 2–3 − 230 − μA tOLP – 54 70 86 ms FB/OLP Clamp Voltage VFB(CLAMP) 4–3 11 12.8 14.0 V OVP Threshold Voltage VCC(OVP) 2–3 27.5 29.5 31.5 V TJ(TSD) − 135 − − °C OLP Delay Time Thermal Shutdown Activating Temperature *VCC(BIAS) > VCC(OFF) always. STR3A100-AN Rev.1.1 SANKEN ELECTRIC CO., LTD. 5 Electrical Characteristics of MOSFET Unless specifically noted, TA is 25°C Characteristic Drain-to-Source Breakdown Voltage Symbol VDSS Test Conditions STR3A15x STR3A15xD Pins 8–1 STR3A16xHD Drain Leakage Current On-Resistance Switching Time Thermal Resistance IDSS RDS(ON) 8–1 Max. Unit 650 – – V 700 – – V – – 300 μA – – 4.0 Ω STR3A152 STR3A152D – – 3.0 Ω STR3A153 STR3A153D – – 1.9 Ω – – 1.4 Ω STR3A155 STR3A155D – – 1.1 Ω STR3A161HD – – 4.2 Ω STR3A162HD – – 3.2 Ω STR3A163HD – – 2.2 Ω 8–1 – – 250 ns – – – 18 °C/W – – – 17 °C/W STR3A154 STR3A154D STR3A151 STR3A151D STR3A152 STR3A152D STR3A153 STR3A153D STR3A16xHD STR3A154 STR3A154D STR3A155 STR3A155D STR3A100-AN Rev.1.1 Typ. STR3A151 STR3A151D 8–1 tf Rθch-C Min. The thermal resistance between channel and case. Case temperature (TC) is measured at the center of the branded side. SANKEN ELECTRIC CO., LTD. 6 Typical Application Circuit C9 CRD Snubber Circuit D1 VAC C5 L2 D4 T1 R3 R9 PC1 P C1 D3 7 6 R8 S C8 C6 R6 5 D2 D/ST D/ST D/ST NC D/ST C4 R4 R5 C7 8 VOUT U2 R2 R7 U1 STR3A100 C2 D GND S/OCP VCC GND FB/OLP C, RC Damper Snubber Circuit 1 ROCP 2 3 4 C3 PC1 The following design features should be observed: • The PCB traces from the D/ST pins (pins 5, 6, 7, and 8) should be as wide as possible, in order to enhance thermal dissipation. • In applications having a power supply specified such that VDS has large transient surge voltages, a clamp snubber circuit of a capacitor-resistor-diode (CRD) combination should be added on the primary winding, P, or a damper snubber circuit of a capacitor (C) or a resistor-capacitor (CR) combination should be added between the D/ST pins and the S/OCP pin. STR3A100-AN Rev.1.1 SANKEN ELECTRIC CO., LTD. 7 Functional Description All of the parameter values used in these descriptions are typical values, according to the STR3A153 specification, unless they are specified as minimum or maximum. With regard to current direction, "+" indicates sink current (toward the IC) and "–" indicates source current (from the IC). D1 T1 VAC C1 Startup Operation Startup Period Figure 2 shows the VCC pin peripheral circuit. The built-in startup circuit is connected to the D/ST pins, and it generates a constant current, ISTARTUP = –2.5 mA to charge capacitor C2 connected to the VCC pin. During this process, when the VCC pin voltage reaches VCC(ON) = 15.3V, the control circuit starts operation. After that, the startup circuit stops automatically, in order to eliminate its own power consumption. The startup time is determined by the C2 capacitance, and a value of 10 to 47 μF is generally recommended. The approximate startup time, tSTART , is calculated as follows: z where: C2 × VCC(ON) – VCC(INT) |I(STARTUP)| VCC 2 STR3A100 GND D2 R2 C2 VD 3 D Figure 2. VCC pin peripheral circuit (1) tSTART is the startup time in s, and ICC ICC(ON) (max) = 2.5 mA VCC(INT) is the initial voltage of the VCC pin in V. In operation, when the VCC pin voltage decreases to VCC(OFF) = 8.1 V, the control circuit stops operation, by the UVLO (Undervoltage Lockout) circuit, and reverts to the state before startup. The rectified voltage from the auxiliary winding, VD (figure 2) becomes a power source to the control circuit after the operation start. Stop Undervoltage Lockout (UVLO) Circuit Figure 3 shows the relationship of VCC and ICC . When the VCC pin voltage increases to VCC(ON) = 15.3 V, the control circuit starts operation and the circuit current, ICC, increases. Start tSTART 5,6,7,8 D/ST P 8.1 V VCC(OFF) 15.3 V VCC pin voltage VCC(ON) Figure 3. VCC versus ICC The VCC pin voltage should become as follows within the specification of input voltage range and the output load range of power supply, taking account of the winding turns of the D winding; the target voltage of the VCC pin voltage is about 15 to 20 V: VCC(BIAS)(max) < VCC < VCC(OVP)(min) (2) 10.5 (V) < VCC < 27.5 (V) STR3A100-AN Rev.1.1 SANKEN ELECTRIC CO., LTD. 8 Bias Assist Function Figure 4 shows the VCC pin voltage behavior during the startup period. When the VCC pin voltage reaches VCC(ON) = 15.3 V, the control circuit starts operation, the circuit current, ICC, increases, and thus the VCC pin voltage begins dropping. At the same time, the auxiliary winding voltage, VD , increases in proportion to the output voltage rise. Thus, the VCC pin voltage is set by the balance between dropping by the increase of ICC and rising by the increase of the auxiliary winding voltage, VD. Just at the turning-off of the power MOSFET, a surge voltage occurs at the output winding. If the feedback control is activated by the surge voltage on light load condition at startup, and the VCC pin voltage decreases to VCC(OFF) = 8.1 V, a startup failure can occur, because the output power is restricted and the output voltage decreases. In order to prevent this, during a state of operating feedback control, when the VCC pin voltage falls to the Startup Current Threshold Biasing Voltage, VCC(BIAS) = 9.5 V, the Bias Assist function is activated. While the Bias Assist function is operating, the decrease of the VCC pin voltage is suppressed by providing the startup current, ISTARTUP , from the startup circuit. VCC pin voltage IC startup VCC(ON) = 15.3 V VCC(BIAS) = 9.5 V VCC(OFF) = 8.1 V Startup failure Time Figure 4. VCC during startup period Without R2 VCC pin voltage By the Bias Assist function, the use of a small value C2 capacitor is allowed, resulting in shortened startup time. Also, because the increase of VCC pin voltage becomes faster when the output runs with excess voltage, the response time of the OVP function can also be shortened. It is necessary to check and adjust the process so that poor starting conditions may be avoided. Auxiliary Winding In actual power supply circuits, there are cases in which the VCC pin voltage fluctuates in proportion to the output of the SMPS (see figure 5), and the Overvoltage Protection (OVP) on the VCC pin may be activated. This happens because C2 is charged to a peak voltage on the auxiliary winding D, which is caused by the transient surge voltage coupled from the primary winding when the power MOSFET turns off. For alleviating C2 peak charging, it is effective to add some value R2, of several tenths of ohms to several ohms, in series with D2 (see figure 6). The optimal value of R2 should be determined using a transformer matching what will be used in the actual application, because the variation of the auxiliary winding voltage is affected by the transformer structural design. STR3A100-AN Rev.1.1 Startup success Target Operating Voltage Increasing by output voltage rising Bias Assist period With R2 IOUT Figure 5. VCC versus IOUT with and without resistor R2 D2 2 VCC STR3A100 Added R2 D C2 GND 3 Figure 6. VCC pin peripheral circuit with R2 SANKEN ELECTRIC CO., LTD. 9 Bobbin Barrier The variation of VCC pin voltage becomes worse if: P1 S1 P2 S2 D • The coupling between the primary and secondary windings of the transformer gets worse and the surge voltage increases (low output voltage, large current load specification, for example). Barrier • The coupling of the auxiliary winding, D, and the secondary side stabilization output winding (winding of the output line which is controlling constant voltage) gets worse and it is subject to surge voltage. In order to reduce the influence of surge voltages on the VCC pin, alternative structures of the auxiliary winding, D, can be used; as examples of transformer structural designs see figure 7. • Winding structural example (a): Separating the auxiliary winding D from the primary side windings P1 and P2. The primary side winding is divided into two windings, P1 and P2. • Winding structural example (b): Placing the auxiliary winding D within the secondary winding S1 in order to improve the coupling of those windings. Winding structural example (a) P1, P2 Primary side winding S1 Secondary side winding, with controlled constant output voltage S2 Secondary side output winding D Auxiliary winding for VCC Bobbin Barrier P1 S1 D S2 S1 P2 The output winding S1 is a stabilized output winding, controlled to constant voltage. Barrier Soft-Start Function Figure 8 shows the behavior of VCC pin voltage and the drain current during the startup period. The IC activates the soft start function during the startup period. The soft start operation period is internally fixed to approximately 7 ms, and the Overcurrent Protection (OCP) threshold voltage steps up in five steps during this period. This reduces the voltage and current stress on the internal power MOSFET and on the secondary-side rectifier. Because the Leading Edge Blanking function (refer to the Constant Output Voltage Control section) is disabled during the soft start period, the on-time may be the LEB time, tBW = 350 ns (280 ns for STR3A16×HD series) or less. It is necessary to check and adjust the OLP delay time and the VCC pin voltage during startup in actual operation. Winding structural example (b) Figure 7. Winding structural examples VCC pin voltage Start up Steady operation VCC(ON) VCC(OFF) Time This ID is limited by OCP operation Drain Current, ID Time Soft-start period with 7 ms fixed internally Figure 8. Soft-start operation waveforms at startup STR3A100-AN Rev.1.1 SANKEN ELECTRIC CO., LTD. 10 Constant Output Voltage Control The constant output voltage control function uses current mode control (peak current mode), which enhances response speed and provides stable operation. This IC compares the voltage, VROCP , of the current detection resistor with the target voltage, VSC, by the internal FB comparator, and controls the peak value of VROCP so that it gets close to VSC. VSC is internally generated by inputting the FB/OLP pin voltage to the feedback control (see the Functional Block diagram) and adding the slope compensation value (refer to figures 9 and 10). • Light load conditions When load conditions become lighter, the output voltage, VOUT , rises, and the feedback current from the error amplifier on the secondary side also increases. The feedback current is sunk at the FB/OLP pin, transferred through a photocoupler, PC1, and the FB/OLP pin voltage decreases. Thus, VSC decreases, the peak value of VROCP is controlled to be low, and 1 VROCP 3 FB/OLP GND S/OCP STR3A100 4 ROCP PC1 the peak drain current of ID increases. This control prevents the output voltage from increasing. • Heavy load conditions When load conditions become greater, the control circuit performs the inverse operation to that described above. Thus, VSC increases and the peak drain current of ID increases. This control prevents the output voltage from decreasing. In the current-mode control method, when the drain current waveform becomes trapezoidal in continuous operating mode, even if the peak current level set by the target voltage is constant, the on-time fluctuates based on the initial value of the drain current. This results in the on-time fluctuating in multiples of the fundamental operating frequency as shown in figure 11. This is called the subharmonics phenomenon. In order to avoid this, the IC incorporates the Slope Compensation function. Because the target voltage is added, a down-slope compensation signal that reduces the peak drain current as the on-duty gets wider relative to the FB/OLP pin signal to compensate VSC , the subharmonics phenomenon is suppressed. Even if subharmonic oscillations occur when the IC has some excess supply being out of feedback control, such as during startup and load shorted, this does not affect performance during normal operation. IFB C3 Figure 9. FB/OLP peripheral circuit VSC – + Target voltage without Slope Compensation Target voltage including Slope Compensation VROCP S/OCP signal voltage across ROCP FB Comparator ton1 Drain Current, ID T Figure 10. Drain current, ID, and FB comparator operation in steady operation STR3A100-AN Rev.1.1 ton2 T T Figure 11. Drain current, ID, waveform in subharmonic oscillation SANKEN ELECTRIC CO., LTD. 11 In the current-mode control method, the FB comparator and/or the OCP comparator may respond to the surge voltage resulting from the drain surge current in turning-on the power MOSFET, and may turn off the power MOSFET irregularly. Leading Edge Blanking, tBW = 350 ns (280 ns for STR3A16×HD series), is built-in to prevent malfunctions caused by surge voltage in turning-on the power MOSFET. However, if the Bias Assist function is always activated during Standby mode, the power loss increases. Therefore, the VCC pin voltage should be more than VCC(BIAS) , for example, by adjusting the turns ratio between the auxiliary winding and secondary winding and/or reducing the value of R2 in figure 6. Automatic Standby Mode Function Automatic standby mode is activated automatically when the drain current, ID , reduces under light load conditions, at which ID is less than 20% to 25% (15% to 20% for STR3A154/54D and STR3A155/55D) of the maximum drain current (it is in the Overcurrent Protection state). The IC modulates its switching frequency randomly within Δf (±4%) superposed on the Average Operation Frequency. The conduction noise with this function is smaller than that without this function, and this function can simplify noise filtering of the input lines of power supply. The operation mode becomes burst oscillation, as shown in figure 12. Burst oscillation reduces switching losses and improves power supply efficiency because of periodic non-switching intervals. Generally, to improve efficiency under light load conditions, the frequency of the burst oscillation becomes just a few kilohertz. Overcurrent Protection Function (OCP) During the transition to burst-oscillation, if the VCC pin voltage decreases to VCC(BIAS) = 9.5 V , the Bias Assist function is activated and stabilizes the Standby mode operation, because ISTARTUP is provided to the VCC pin so that the VCC pin voltage does not decrease to VCC(OFF). Random Switching Function Overcurrent Protection Function (OCP) detects each peak drain current level of the power MOSFET on pulse-by-pulse basis, and limits the output power. This function incorporates the Input Compensation function to reduce OCP point variation for the AC input voltage, without any additional external components. This OCP function detects the drain current by the current detection resistor, ROCP , which is connected between the S/OCP pin and the GND pin. When the voltage drops on both sides of ROCP increase to an internal OCP threshold voltage, the power MOSFET is turned off. Burst Oscillation mode Output Current, IOUT Less than a few kilohertz Drain Current, ID Normal Load Standby Load Normal Load Figure 12. Automatic standby mode operation STR3A100-AN Rev.1.1 SANKEN ELECTRIC CO., LTD. 12 ICs with PWM control usually have some detection delay time on OCP detection. The steeper the slope of the actual drain current at a high AC input voltage is, the later the actual detection point is, compared to the internal OCP threshold voltage, VOCP. Thus, the actual OCP point limiting the output current usually has some variation depending on the AC input voltage, as shown in figure 13. Overvoltage Protection Function (OVP) When the voltage between the VCC pin and the GND pin increases to VCC(OVP) , 29.5 V or more, the OVP function is activated and stops switching operation. The IC has two operation types of OVP function. One is the latched shutdown, the other is auto restart. The IC incorporates a built-in Input Compensation function that superposes a signal with a defined slope onto the detection signal on the S/OCP pin as shown in figure 14. When AC input voltage is lower and the duty cycle is longer, the OCP compensation level increases. Thus the OCP point in low AC input voltage increases to minimize the difference of OCP points between low AC input voltage and high AC input voltage. • Latched Shutdown type: STR3A100 series. When the OVP function is activated, the IC stops switching operation. The VCC pin voltage decreases to VCC(BIAS) = 9.5 V, and then the Bias Assist function is activated. Because the Bias Assit function prevents the VCC pin voltage from decreasing to VCC(OFF) = 8.1 V, by applying the startup current, the IC remains in latched state. Releasing the latched state is done by turning off the input voltage and by dropping the VCC pin voltage below VCC(OFF) . Because the compensation signal level is designed to depend upon the on-time of the duty cycle, the OCP threshold voltage after compensation, VOCP(ontime) , is calculated as below. When the duty cycle becomes 36% or more, the OCP threshold voltage after compensation remains at VOCP(H) = 0.88 V, constantly. VOCP(ontime) (V) = VOCP(L)(V) + DPC (mV/μs) × On Time (μs). where: VOCP(L) is the OCP threshold voltage at zero duty cycle (V), DPC is the OCP compensation coefficient (mV/μs), and On Time is the on-time of the duty cycle (μs): On Time = (D / fOSC(AVG)) (3) • Auto Restart type: STR3A100D and STR3A100HD series. While the OVP function is active, because the Bias Assist function is disabled, the VCC pin voltage falls below VCC(OFF) . At that time, the UVLO (Undervoltage Lockout) circuit becomes active, stopping the control circuit and then the IC reverts to the state before startup. Then, when the VCC pin voltage rises due to the startup current and reaches VCC(ON) = 15.3 V, the control circuit will return to normal operation again. In this manner, the intermittent oscillation mode is operated by the UVLO circuit repeatedly while there is an excess voltage condition. By this intermittent oscillation operation, stress on the internal and external circuits, such as the power MOSFET and the secondary rectifier diode, is reduced. Furthermore, because the switching period is shorter than an oscillation stop period, power consumption under intermittent operation can be minimized. When the fault condition is removed, the IC returns to normal operation automatically. 265 VAC (as an example) 85 VAC (as an example) 0.88 V 0.9 Variance resulting from propagation delay t npu Ci A ut Low inp AC h Hig Output Current, IOUT Figure 13. Output current at OCP without input compensation STR3A100-AN Rev.1.1 About 0.82 VOCP(ontime) Typical (V) Output Voltage, VOUT 0.5 0 0 15 36 80 100 Duty Cycle, D (%) Figure 14. Relationship of duty cycle and VOCP after compensation SANKEN ELECTRIC CO., LTD. 13 When the auxiliary winding supplies the VCC pin voltage, the OVP function is able to detect an excessive output voltage, such as when the detection circuit for output control is open on the secondary side, because the VCC pin voltage is proportional to the output voltage. mode operation by the UVLO circuit is performed repeatedly. When the fault condition is removed, the IC returns to normal operation automatically. The output voltage of the secondary side at OVP operation, VOUT(OVP) , is calculated approximately as follows: If the temperature of the Control Part of the IC reaches more than the Thermal Shutdown Activating Temperature TJ(TSD) = 135°C (min), the Thermal Shutdown function (TSD) is activated. VOUT(OVP) = VOUT(normal operation) × 29.5 (V) VCC(normal operation) (4) Overload Protection Function (OLP) When the peak drain current of ID is limited by OCP operation, the output voltage, VOUT , decreases and the feedback current from the secondary photo-coupler, IFB (see figure 15), becomes zero. As a result, the FB/OLP pin voltage increases. When the FB/OLP pin voltage increases to VFB(OLP) = 8.1 V, or more, and remains at that level for the OLP Delay Time, tOLP = 70 ms, or more, the OLP function is activated. It stops switching operation and reduces stress on the power MOSFET, secondary rectifier, and so on. When the OLP function is activated, the Bias Assist function is disabled, as mentioned in the auto restart type description of the Overvoltage Protection Function (OVP) section, and intermittent Thermal Shutdown Function (TSD) The IC has two operation types of TSD function. One is the latched shutdown, the other is auto restart. These types perform by the same operations as mentioned in the Overvoltage Protection Function (OVP) section. • Latched Shutdown type: STR3A100 series. When the TSD function is active, the IC stops switching operation in latched state. Releasing the latched state is done by turning off the input voltage and by dropping the VCC pin voltage below VCC(OFF). • Auto Restart type: STR3A100D, STR3A100HD series. Intermittent mode operation by the UVLO circuit is performed repeatedly. When the factor causing the overheating condition is removed, and the temperature of the Control Part falls below TJ(TSD) , the IC returns to normal operation automatically. Switching turns off VCC Pin Voltage VCC(OFF)= 8.1V VFB(OLP)= 8.1V FB/OLP Pin Voltage Drain Current, GND FB/OLP Switching stopped interval 3 4 PC1 OLP Delay Time, tOLP ID C3 IFB Figure 15. OLP operation waveforms (left), and FB/OLP pin peripheral circuit (right) STR3A100-AN Rev.1.1 SANKEN ELECTRIC CO., LTD. 14 Design Notes Peripheral Components Take care to use the proper rating and proper type of components. • Input and output electrolytic capacitors ▫ Apply proper design margin to accommodate ripple current, voltage, and temperature rise. ▫ Use of high ripple current and low impedance types, designed for switch-mode power supplies, is recommended, depending on their purposes. • Transformer ▫ Apply proper design margin to core temperature rise by core loss and copper loss. ▫ Because the switching circuits contain high frequency currents, the skin effect may become a consideration. ▫ In consideration of the skin effect, choose a suitable wire gauge in consideration of the rms current and a current density of about 3 to 4 A/mm2. ▫ If measures to further reduce temperature are still necessary, use paralleled wires or litz wires to increase the total surface area of the wiring. • Current detection resistor, ROCP ▫ A high frequency switching current flows to ROCP , and may cause poor operation if a high inductance resistor is used. ▫ Choose a low inductance and high surge-tolerant type. Phase Compensation A typical phase compensation circuit with a secondary shunt regulator (U2) is shown in figure 16. The values for C7and R6 are recommended to be about 0.047 to 0.47 μF, and about 4.7 to 470 kΩ, respectively, and should be selected based on actual operation in the application. Place C3 between the FB/OLP pin and the GND pin, as shown in figure 17, to perform high frequency noise reduction and phase compensation. The value for C3 is recommended to be about 2200 pF to 0.01 μF, and should be selected based on actual operation in the application. PCB Trace Layout and Component Placement PCB circuit trace design and component layout significantly affect operation, EMI noise, and power dissipation. Therefore, pay extra attention to these designs. In general, where high frequency current traces form a loop, as shown in figure 18, wide, short traces, and small circuit loops are important to reduce line impedance. In addition, earth ground traces affect radiated EMI noise, and the same measures should be taken into account. Switch-mode power supplies consist of current traces with high frequency and high voltage, and thus trace design and component layouts should be done to comply with all safety guidelines. Furthermore, because the incorporated power MOSFET has a positive thermal coefficient of RDS(ON) , consider it when preparing a thermal design. 8 7 D2 STR3A100 C2 R2 T1 D S/OCP VCC GND FB/OLP 2 ROCP T1 5 D/ST D/ST D/ST NC D/ST 1 3 4 C3 PC1 L2 D4 6 VOUT Figure 17. FB/OLP peripheral circuit R9 R4 PC1 R8 C6 S R5 C8 C7 U2 R6 R7 GND Figure 16. Peripheral circuit around secondary shunt regulator (U2) STR3A100-AN Rev.1.1 Figure 18. High-frequency current loops (hatched areas) SANKEN ELECTRIC CO., LTD. 15 Figure 19 shows a circuit layout design example. • ROCP Trace Layout • S/OCP Trace Layout: S/OCP pin to ROCP to C1 to T1 (winding P) to D/ST pin ROCP should be placed as close as possible to the S/OCP pin. The connection between the power ground of the main trace and the control circuit ground should be at a single point ground (A in figure 19) to remove common impedance, and to avoid interference from switching currents to the control circuit. Figure 19 also shows a circuit layout design example for the secondary side. This is the main trace containing switching currents, and thus it should be as wide and short as possible. If C1 and the IC are distant from each other, an electrolytic capacitor or film capacitor (about 0.1 μF and with proper voltage rating) near the IC or the transformer is recommended to reduce impedance of the high frequency current loop. • Secondary Smoothing Circuit Trace Layout: T1 (winding S) to D4 to C6 • GND Trace Layout: GND pin to C2 (negative pin) to T1 (winding D) to R2 to D2 to C2 (positive pin) to VCC pin This trace should be as wide as possible. If the loop distance is lengthy, leakage inductance resulting from the long loop may increase surge voltage at turning off the incorporated power MOSFET. Proper secondary trace layout helps to increase margin against the power MOSFET breakdown voltage, and reduces stress on the clamp snubber circuit and losses in it. This trace also must be as wide and short as possible. If C2 and the IC are distant from each other, placing a capacitor (approximately 0.1 to 1.0 μF film capacitor) close to the VCC pin and the GND pin is recommended. C9 D4 T1 R3 C5 C1 P D3 8 7 6 S 5 D2 D/ST D/ST D/ST NC D/ST C4 C6 R2 U1 STR3A100 C2 D Main power circuit trace S/OCP VCC GND FB/OLP 1 2 ROCP 3 4 GND trace for the IC C3 PC1 A Figure 19. Peripheral circuit example around the IC STR3A100-AN Rev.1.1 SANKEN ELECTRIC CO., LTD. 16 • The contents in this document are subject to changes, for improvement and other purposes, without notice. Make sure that this is the latest revision of the document before use. • Application and operation examples described in this document are quoted for the sole purpose of reference for the use of the products herein and Sanken can assume no responsibility for any infringement of industrial property rights, intellectual property rights or any other rights of Sanken or any third party which may result from its use. • Although Sanken undertakes to enhance the quality and reliability of its products, the occurrence of failure and defect of semiconductor products at a certain rate is inevitable. Users of Sanken products are requested to take, at their own risk, preventative measures including safety design of the equipment or systems against any possible injury, death, fires or damages to the society due to device failure or malfunction. • Sanken products listed in this document are designed and intended for the use as components in general purpose electronic equipment or apparatus (home appliances, office equipment, telecommunication equipment, measuring equipment, etc.). When considering the use of Sanken products in the applications where higher reliability is required (transportation equipment and its control systems, traffic signal control systems or equipment, fire/crime alarm systems, various safety devices, etc.), and whenever long life expectancy is required even in general purpose electronic equipment or apparatus, please contact your nearest Sanken sales representative to discuss, prior to the use of the products herein. The use of Sanken products without the written consent of Sanken in the applications where extremely high reliability is required (aerospace equipment, nuclear power control systems, life support systems, etc.) is strictly prohibited. • In the case that you use Sanken products or design your products by using Sanken products, the reliability largely depends on the degree of derating to be made to the rated values. Derating may be interpreted as a case that an operation range is set by derating the load from each rated value or surge voltage or noise is considered for derating in order to assure or improve the reliability. In general, derating factors include electric stresses such as electric voltage, electric current, electric power etc., environmental stresses such as ambient temperature, humidity etc. and thermal stress caused due to self-heating of semiconductor products. For these stresses, instantaneous values, maximum values and minimum values must be taken into consideration. In addition, it should be noted that since power devices or IC's including power devices have large self-heating value, the degree of derating of junction temperature affects the reliability significantly. • When using the products specified herein by either (i) combining other products or materials therewith or (ii) physically, chemically or otherwise processing or treating the products, please duly consider all possible risks that may result from all such uses in advance and proceed therewith at your own responsibility. • Anti radioactive ray design is not considered for the products listed herein. • Sanken assumes no responsibility for any troubles, such as dropping products caused during transportation out of Sanken's distribution network. • The contents in this document must not be transcribed or copied without Sanken's written consent. STR3A100-AN Rev.1.1 SANKEN ELECTRIC CO., LTD. 17