This engineering report describes a 61 V (nominal), 300 W reference design for a power supply operating from 90 VAC to 132 VAC. A 5 V, 4 A standby output is also provided. The power supply main output is designed with a constant voltage / constant current characteristic for use in battery charger applications.
The design is based on the TFS7707H operating from doubled mains, with no PFC input stage. It can run at maximum power without fan at 115 VAC, room temperature, but will require forced air for low line and/or elevated ambient temperatures.
The schematic shows a 2-switch forward power supply with flyback standby utilizing the TFS7707H, powered via a voltage doubler. The secondary control circuitry provides CV/CC control for use in battery charger applications.
EMI Filtering/Voltage Doubler
Capacitors C1 and C2 are used to control differential mode noise. Resistors R1-3 discharge C1 and C2 when AC power is removed. Inductors L1 and L2 primarily control common mode EMI, and to some extent, differential mode EMI. The heat sink for U1 is connected to primary return to eliminate the heat sink as a source of radiated/capacitive coupled noise. Thermistor RT1 provides inrush limiting. Capacitors C10 an C26 filter common mode EMI. Capacitors C3 and C4, along with BR1, form a voltage doubler to provide a ~250-380 VDC B+ supply from the 90-132 VAC input. Capacitor C27 provides local HV bypassing for the 5 V standby converter.
Primary Bias Supply
The standby supply utilizes built-in capability of the U1 HiperTFS-2 device. Components D15 and C28 provide a 15 V (nominal) flyback bias supply for U1 generated from a primary-referred winding on standby transformer T2. Components D17 and C30 generate a 12 V bias supply for the secondary control circuitry via a secondary-referred winding on T2. Components R42, R44, and VR5-6 clamp the primary and secondary VCC output voltages when the 5 V standby supply is heavily loaded. Components Q3, R31, and C20 comprise a "capacitor multiplier" circuit to provide extra ripple filtering for the secondary VCC supply.
Main Forward Converter/Standby
The schematic depicts a 61 V, 280 W Forward DC-DC converter with constant voltage/constant current output implemented using the TFS7707H.
Integrated circuit U1 incorporates the control circuitry, drivers and output MOSFETs necessary for a 2-switch forward converter and a flyback standby converter.
Components D6, C11, R19-20, and VR4 form a turn-off clamping circuit that limits the standby drain voltage of U1. Zener VR4 provides a defined clamp voltage and maintains a maximum voltage (150 V) on clamp capacitor C11 for higher light/no-load efficiency.
Diode D5 provides initial biasing for the main converter high-side driver in U1. Subsequent power is supplied by a winding on T1, recitified and filtered by D3, C6-7 and R15.
The output of transformer T1 is rectified and filtered by D7, L3, and C13-14. Output rectifier D7 is a 300 V rectifier chosen for high efficiency. A snubber consisting of R21 and C12 helps limit the peak voltage excursion on the output rectifier.
Output Current and Voltage Control
Output current is sensed via resistors R39 and R40. These resistors are clamped by diode D14 to avoid damage to the current control circuitry during an output short-circuit. Components R32 and U2 provide a voltage reference for current sense and voltage sense amplifiers U3A and U3B. The reference voltage for current sense amplifier U3A is divided down by R34-35 and R38. The default current limit setting is 4.589 A, as programmed by R39-40, R34-35, and R38. Voltage from the current sense resistors is applied to the inverting input of U3A via R36. Opamp U3A drives optocoupler U4 through D8 and R23. Components R23, R33, R36, R37, C23, and C25 are used for frequency compensation of the current loop.
A 5 V, 4 A standby output is provided via a triple insulated winding on standby transformer T2. This winding is rectified and filtered by D16, and C31-32. Components L4 and C35 provide additional filtering to remove high frequency ripple and noise. Snubber C29 and R43 helps limit the peak voltage excursion on D16. The 5 V output is divided down by R49 and R51, and is applied to the reference input of error amplifier U7, which controls the standby section of U1 via R45 and U6. Resistor R46 provides bias current to U7, while C33 and R47 comprise a soft-finish network to eliminate output voltage overshoot at start-up. Components R45, R48, and C34 compensate the standby control loop.