Application Information STR2W100D Series PWM Off-Line Switching Regulators General Description STR2W100D 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 < 25 mW at no load ▫ Normal load operation: PWM switching ▫ Light load operation: Standby mode (Burst oscillation) • Soft Start function: reduces stress on internal power MOSFET and output rectifier diode • Protection Functions: ▫ Overcurrent Protection function (OCP); Pulse-by-pulse, built-in compensation circuit to minimize OCP point variation on AC input voltage ▫ Overload Protection function (OLP); Auto restart, built-in timer, reduces heat during overload condition, and no external components required Figure 1. STR2W100D series packages are fully molded TO-220 package types. Pin 2 is deleted for greater isolation. ▫ Overvoltage Protection function (OVP); Auto restart ▫ Thermal Shutdown function (TSD); Auto restart Applications Switching power supplies for electronic devices such as: • Home appliances • Digital appliances • Office automation (OA) equipment • Industrial apparatus • Communication facilities The product lineup for the STR2W100D series provides the following options: POUT* (W) MOSFET Part Number STR2W152D STR2W153D fOSC (kHz) 67 VDSS(min) (V) 650 RDS(on) (max) (Ω) 230 VAC 85 to 265 VAC 3.0 60 40 1.9 90 60 *The listed output power is based on the package 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. STR2W100D-AN Rev.3.1 SANKEN ELECTRIC CO., LTD. Functional Block Diagram 4 VCC Startup UVLO 7 Reg VREG OVP D/ST 1 TSD NC PWM Oscillator DRV SQ R OCP VCC Drain peak current compensation OLP Feedback control FB/OLP 6 Slope compensation Pin-out Diagram D/ST S/OCP 1 3 VCC GND 4 5 FB/OLP NC 3 S/OCP GND 5 STR2W100D Pin List Table Number Name 1 D/ST 3 S/OCP Function MOSFET drain pin and input of the startup current MOSFET source and input of Overcurrent Protection (OCP) signal 4 VCC Power supply voltage input for Control Part and input of Overvoltage Protection (OVP) signal 5 GND Ground 6 FB/OLP 7 NC 6 7 LEB Feedback signal input for constant voltage control signal and input of Overload Protection (OLP) signal No connection Table of Contents Functional Description 8 Startup Operation 8 Startup Period Undervoltage Lockout (UVLO) Circuit Bias Assist Function Auxiliary Winding Soft-Start Function Constant Output Voltage Control Automatic Standby Mode Function STR2W100D-AN Rev.3.1 8 8 9 9 10 11 12 Random Switching Function Overcurrent Protection Function (OCP) Overvoltage Protection Function (OVP) Overload Protection Function (OLP) Thermal Shutdown Function (TSD) Design Notes 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 Outline Drawing, TO-220F-6L 10.0±0.2 4.2±0.2 Gate burr Ø3.2±0.2 7.9±0.2 16.9±0.3 4±0.2 0.5 2.8±0.2 STR a b 5.0±0.5 6-0.74±0.15 6-0.65 +0.2 -0.1 (5.4) R-end ) -R1 (2 6×P1.27±0.15=7.62±0.15 10.4±0.5 2.8 2.6±0.1 Dimensions from root 0.45 +0.2 -0.1 Dimensions between roots 5.08±0.6 Dimensions between tips 0.5 1 2 3 4 5 6 7 Unit: mm Dashed line at gate burr indicates protrusion of 0.3 mm (maximum) Leadform: LF2003 Pin 2 is deleted in order to ensure creepage and clearance of the high voltage pin (pin 1) and low voltage pin (pin 3) 0.5 Front view 0.5 0.5 Side view a. Type Number: 2W1xxD b. Lot Number: 1st and 2nd letter: Control number 3rd letter: Last digit of year (0-9) 4th letter: Month 1 to 9 for Jan. to Sept. O for Oct. N for Nov. D for Dec. 5th and 6th letter: Day of month (01-31) 7th and 8th letter: Control number Pin treatment Pb-free. Device composition compliant with the RoHS directive. STR2W100D-AN Rev.3.1 SANKEN ELECTRIC CO., LTD. 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 Symbol Drain Peak Current IDPEAK Maximum Switching Current IDMAX EAS Avalanche Energy ILPEAK Notes STR2W152D STR2W153D STR2W152D STR2W153D Pins Single pulse 1−3 TA = −20°C to 125°C 1−3 STR2W152D STR2W153D STR2W152D Single pulse, VDD = 99 V, L = 20 mH 1−3 STR2W153D S/OCP Pin Voltage VOCP 3−5 Rating Unit 6.0 A 9.5 A 6.0 A 9.5 A 62 mJ 86 mJ 2.3 A 2.7 A −2 to 6 V Control Part Input Voltage VCC 4−5 32 V FB/OLP Pin Voltage VFB 6−5 −0.3 to 14 V FB/OLP Pin Sink Current IFB 6−5 STR2W152D MOSFET Power Dissipation PD1 STR2W153D With infinite heatsink 1.0 mA 23.8 W 26.5 W 1.3 W 4−5 0.13 W −20 to 115 °C 1−3 Without heatsink VCC × ICC Control Part Power Dissipation PD2 Internal Frame Temperature In Operation* TF – Operating Ambient Temperature TOP – −20 to 115 °C Storage Temperature Tstg – −40 to 125 °C Channel Temperature Tch – 150 °C *The recommended internal frame temperature, TF , is 105°C (max). STR2W100D-AN Rev.3.1 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) Test Conditions VCC = 12 V Pins Min. Typ. Max. Unit 4–5 13.8 15.3 16.8 V 4–5 7.3 8.1 8.9 V 4–5 − − 2.5 mA Minimum Start Voltage VST(ON) 4–5 − 40 − V Startup Current ISTARTUP VCC = 13.5 V 4–5 −3.9 −2.5 −1.1 mA Startup Current Threshold Biasing Voltage* VCC(BIAS) ICC = –100 μA 4–5 8.5 9.5 10.5 V Average Operation Frequency fOSC(AVG) 1–5 60 67 74 kHz Frequency Modulation Deviation Maximum Duty Cycle Leading Edge Blanking Time Δf 1–5 − 5 − kHz DMAX 1–5 65 74 83 % tBW – − 390 − ns OCP Compensation Coefficient DPC – − 17 − mV/μs OCP Compensation Duty Cycle Limit DDPC – − 36 − % OCP Threshold Voltage at Zero Duty Cycle VOCP(L) 3–5 0.69 0.78 0.87 V OCP Threshold Voltage at 36% Duty Cycle VOCP(H) 3–5 0.79 0.88 0.97 V Maximum Feedback Current IFB(MAX) Minimum Feedback Current IFB(MIN) FB/OLP Oscillation Stop Threshold Voltage VFB(OFF) OLP Threshold Voltage OLP Operation Current 6–5 −280 −170 −90 μA 6–5 −30 −15 −7 μA VCC = 32 V 6–5 1.3 1.4 1.5 V VFB(OLP) VCC = 32 V 6–5 7.3 8.1 8.9 V ICC(OLP) VCC = 12 V 4–5 − 230 − μA tOLP 1–5 54 68 82 ms FB/OLP Clamp Voltage VFB(CLAMP) 6–5 11 12.8 14 V OVP Threshold Voltage VCC(OVP) 4–5 26 29 32 V TJ(TSD) − 130 − − °C OLP Delay Time Thermal Shutdown Activating Temperature VCC = 12 V *VCC(BIAS) > VCC(OFF) always. STR2W100D-AN Rev.3.1 SANKEN ELECTRIC CO., LTD. 5 Electrical Characteristics of MOSFET Unless specifically noted, TA is 25°C Characteristic Symbol Drain-to-Source Breakdown Voltage VDSS Drain Leakage Current IDSS On-Resistance RDS(ON) Switching Time tf Thermal Resistance* Rθch-F Test Conditions STR2W152D STR2W153D Pins Min. Typ. Max. Unit 1–5 650 – – V 1–5 – – 300 μA – – 3.0 Ω – – 1.9 Ω – – 250 ns 1–5 1–5 STR2W152D STR2W153D – – – 2.48 °C/W – – 1.95 °C/W *The thermal resistance between the channels of the MOSFET and the internal frame. STR2W100D-AN Rev.3.1 SANKEN ELECTRIC CO., LTD. 6 Typical Application Circuit CRD Clamp Snubber Circuit D1 VAC C5 L2 D4 T1 R3 R9 PC1 P C1 D3 R8 S C8 C6 C7 D2 U2 R2 D R6 R7 GND D/ST 2 S/OCP V CC GND FB/OLP NC C2 R4 R5 U1 STR 2W100D VOUT 1 C4 C, RC Damper Snubber Circuit 3 4 5 6 7 C3 PC1 ROCP C9 The following design features should be observed: • The PCB traces from the D/ST pin 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 (RC) combination should be added between the D/ST pin and the S/OCP pin. STR2W100D-AN Rev.3.1 SANKEN ELECTRIC CO., LTD. 7 Functional Description All of the parameter values used in these descriptions are typical values, according to the STR2W153D 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 pin, 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. 1 D/ST VCC D2 4 STR2W100D GND P R2 C2 5 D The approximate startup time, tSTART , is calculated as follows: tSTART z C2 × where: VCC(ON) – VCC(INT) |ISTARTUP| (1) Figure 2. VCC pin peripheral circuit tSTART is the startup time in s, and C2 should be an electrolytic capacitor, in the range 10 to 47 μF, for general power supply applications. ICC ICC(ON) (max) = 2.5 mA 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 voltage from the auxiliary winding, D, in figure 2 becomes a power source to the control circuit after the operation start. The auxiliary winding voltage is targeted to be about 15 to 20 V, taking account of the winding turns of the D winding, so that the VCC pin voltage should become as follows within the specification of input voltage range and the output load range of power supply: VCC(BIAS)(max) < VCC < VCC(OVP)(min) 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 VCC(INT) is the initial voltage of the VCC pin in V. 8.1 V VCC(OFF) 15.3 V VCC pin voltage VCC(ON) Figure 3. VCC versus ICC (2) 10.5 (V) < VCC < 26.0 (V) STR2W100D-AN Rev.3.1 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 increases in proportion to the output voltage rise. And 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. 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 voltage is suppressed by a supplementary current from the Startup circuit. VCC pin voltage Target Operating Voltage Increasing by output voltage rising Bias Assist period 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 shortening 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). 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. Startup success IC startup With R2 IOUT Figure 5. VCC versus IOUT with and without resistor R2 D2 4 VCC STR2W100D Added R2 D C2 GND 5 Figure 6. VCC pin peripheral circuit with R2 STR2W100D-AN Rev.3.1 SANKEN ELECTRIC CO., LTD. 9 Bobbin Barrier P1 S1 P2 S2 D The variation of VCC pin voltage becomes worse if: • 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 designs for 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. P1, P2 Primary side winding S1 Secondary side winding, of which the output voltage is controlled constant S2 Secondary side output winding D Auxiliary winding for VCC Bobbin Barrier • Winding structural example (b): Placing the auxiliary winding D within the secondary winding S1 in order to improve the coupling of those windings. P1 S1 D S2 S1 P2 The output winding S1 is a stabilized output winding, controlled to constant voltage. 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 = 390 ns or less. It is necessary to check and adjust the OLP delay time and the VCC pin voltage during startup in actual operation. Barrier Figure 7. Winding structural examples VCC pin voltage Start up Steady operation VCC(ON) VCC(OFF) Time Drain Current, ID This ID is limited by OCP operation Time Soft-start period with 7 ms fixed internally Figure 8. Soft-start operation waveforms at startup STR2W100D-AN Rev.3.1 SANKEN ELECTRIC CO., LTD. 10 Constant Output Voltage Control For enhanced response speed and stability, current mode control (peak current mode control) is used for constant voltage control of the output voltage. This IC compares the voltage, VROCP , of a 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 generated by inputting the FB/ OLP pin voltage to the feedback control (see 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 3 5 FB/OLP GND S/OCP STR2W100D 6 ROCP VROCP PC1 the peak drain current, ID , decreases. 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. It is called the subharmonics phenomenon. In order to suppress the subharmonics phenomenon, the IC incorporates a slope compensation signal to the target voltage, Vsc. Because the compensation signal is a down slope signal, Vsc output on the FB/OLP pin goes down as the duty cycle rises, reducing the controlled drain peak current. 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 – VSC VROCP + V ROCP 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 STR2W100D-AN Rev.3.1 Target voltage without Slope Compensation Target voltage including Slope Compensation 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 (390 ns), is built-in to prevent malfunctions caused by surge voltage in turning-on the power MOSFET. 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 25% to 30% of the maximum drain current (it is in the Overcurrent Protection state). 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. 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). 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. Random Switching Function The IC modulates its switching frequency randomly within Δf (5 kHz) superposed on the Average Operation Frequency, fOSC(AVG) = 67 kHz. 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. Overcurrent Protection Function (OCP) 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 the OCP threshold voltage, VOCP , 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 STR2W100D-AN Rev.3.1 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 . And thus the actual OCP point limiting the output current usually has some variation depending on the AC input voltage, as shown in figure 13. 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. Because the compensation signal level is designed to depend upon the on-time of the duty cycle, OCP threshold voltage after compensation, VOCP(ontime) , is calculated as below. When the duty cycle becomes 36% or more, 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). (3) 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 the on-time of the duty cycle (μs): On Time = (D / fOSC(AVG) ) 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. The output voltage of the secondary side at OVP operation, VOUT(OVP) , is calculated approximately as follows: VOUT(OVP) = VOUT(normal operation) × 29 (V) VCC(normal operation) Variance resulting from propagation delay IOUT Figure 13. Output current at OCP without input compensation (4) 265VAC (as an example) 85VAC (for example) 0.88 V 0.9V About 0.82V ut inp C A t Low npu Ci A h Hig STR2W100D-AN Rev.3.1 While OVP is active, because the Bias Assist function is disabled, the VCC pin voltage falls below the Operation Stop Voltage, VCC(OFF) (8.1 V). At that time, the UVLO (Undervoltage Lockout) circuit becomes active, stopping the control circuit and then restarting it. Then, when the VCC pin voltage rises due to the startup current and reaches the Operation Start Voltage, 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. VOCP(ontime) (Typical) (V) VOUT Overvoltage Protection Function (OVP) When the voltage between the VCC pin and the GND pin increases to VCC(OVP) = 29 V or more, the OVP function is activated and stops switching operation. 0.5V 0 0% 15% 36% 80% Duty Cycle, D (%) 100% Figure 14. Relationship of duty cycle and VOCP at fOSC(AVG) = 67 kHz SANKEN ELECTRIC CO., LTD. 13 Overload Protection Function (OLP) When the drain peak current 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 (68 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 Overvoltage Protection Function (OVP) section, and intermittent mode operation by the UVLO circuit is performed repeatedly. When the fault condition is removed, the IC returns to normal operation automatically. Thermal Shutdown Function (TSD) If the temperature of the Control Part of the IC reaches more than the Thermal Shutdown Activating Temperature TJ(TSD) = 130°C (min), the Thermal Shutdown function (TSD) is activated. During TSD operation, the Bias Assist function is disabled, and intermittent mode operation by the UVLO circuit is performed repeatedly. If 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, Switching stopped interval GND FB/OLP 5 6 PC1 OLP Delay Time, tOLP ID C3 IFB Figure 15. OLP operation waveforms (left), and FB/OLP pin peripheral circuit (right) STR2W100D-AN Rev.3.1 SANKEN ELECTRIC CO., LTD. 14 Design Notes Peripheral Components Take care to use properly rated and proper type of components. compensation. The value for C3 is recommended to be about 2200 pF to 0.01 μF. • Input and output electrolytic capacitors ▫ Apply proper design margin to 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 switching currents 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 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. C3 should be connected close to the FB/OLP pin and the GND pin, 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. D2 Phase Compensation A typical phase compensation circuit with a secondary shunt regulator (U2) is shown in figure 16. The value for C7 is recommended to be about 0.047 to 0.47 μF, 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 1 D/ST NC 4 V CC C2 R2 T1 D STR2W100D S/OCP 3 ROCP GND FB/OLP 5 6 C3 PC 1 L2 D4 T1 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 STR2W100D-AN Rev.3.1 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 main trace and the control circuit ground should be connected by a single point ground (A in figure 19) to remove common impedance, and to avoid interference from switching currents to the control circuit. 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. Figure 19 also shows a circuit layout design example for the secondary side. • GND Trace Layout: GND pin to C2 (negative pin) and T1 (winding D) to R2 to D2 to C2 (positive pin) to VCC pin This trace should be as wide as possible. (1) Secondary Smoothing Circuit Trace Layout: T1 (winding S) to D4 to C6 This trace also must be as wide and short as possible. If the loop distance is lengthy, leakage inductance resulting from the long loop may increase surge voltage at turning off a power MOSFET. If C2 and the IC are distant from each other, placing a capacitor (approximately 0.1 to 1.0 μF (50 V) film capacitor) close to the VCC pin and the GND pin is recommended. 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. D4 T1 C5 R3 P C1 D3 S D2 D/ST 2 S/OCP VCC GND FB/OLP NC STR 2W100D 1 C2 R2 D C3 Main power circuit trace GND trace for the IC 3 4 5 6 7 C4 C6 PC 1 ROCP C9 A Figure 19. Peripheral circuit example around the IC STR2W100D-AN Rev.3.1 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. STR2W100D-AN Rev.3.1 SANKEN ELECTRIC CO., LTD. 17