20 W InnoSwitch-EP Dual Output Supply

By Power Integrations |

This document is an engineering report describing a 1.5 A, 12 V and 0.5 A, 5 V dual output embedded power supply utilizing INN2605K from the InnoSwitch-EP family of ICs. This design shows the high power density and efficiency that is possible due to the high level of integration while still providing exceptional performance.

Figure 1 Populated Circuit Board
Figure 2 Schematic Diagram

Input EMI Filtering

Fuse F1 isolates the circuit and provides protection from component failure and the common mode chokes L1 and L2 with capacitors, C9 and C12, provides attenuation for EMI. Bridge rectifier BR1 rectifies the AC line voltage and provides a full wave rectified DC across the filter consisting of C1 and C2. There is no need to use an inrush current limiter in the circuit with the high peak forward surge current rated bridge rectifier, GBL06. The differential inductance of common mode choke L1 with capacitors C1 and C2 provide differential noise filtering.

InnoSwitch-Ep Primary

One side of the transformer primary is connected to the rectified DC bus, the other is connected to the integrated 725 V power MOSFET inside the InnoSwitch-EP IC (U1).

A low cost RCD clamp formed by D1, R1, R2, and C3 limits the peak drain voltage due to the effects of transformer leakage reactance and output trace inductance.

The IC is self-starting, using an internal high-voltage current source to charge the BPP pin capacitor, C4, when AC is first applied. During normal operation the primary side block is powered from an auxiliary winding on the transformer. The output of this is configured as a flyback winding which is rectified and filtered using diode D2 and capacitor C5, and fed in the BPP pin via a current limiting resistor R3. Radiated EMI caused by resonant ringing across diode D2 is reduced via snubber components R11 and C14. The primary side overvoltage protection is obtained using Zener diode VR1. In the event of overvoltage at output, the increased voltage at the output of the bias winding cause the Zener diode VR1 to conduct and triggers the OVP latch in the primary side controller of the InnoSwitch-EP IC.

Resistor R17 and R18 provide line voltage sensing and provide a current to U1, which is proportional to the DC voltage across capacitor C2. At approximately 100 V DC, the current through these resistors exceeds the line under-voltage threshold, which results in enabling of U1. At approximately 460 V DC, the current through these resistors exceeds the line over-voltage threshold, which results in disabling of U1.

InnoSwitch-EP IC Secondary

The secondary side of the InnoSwitch-EP provides output voltage, output current sensing and drive to a MOSFET providing synchronous rectification.

Output rectification for the 5 V output is provided by SR FET Q2. Very low ESR capacitor C16 provides filtering, and inductor L4 and capacitor C18 form a second stage filter that significantly attenuates the high frequency ripple and noise at the 5 V output.

Output rectification for the 12 V output is provided by SR FET Q1. Very low ESR capacitors C10 provides filtering, and Inductor L5 and capacitor C13 form a second stage filter that significantly attenuates the high frequency ripple and noise at the 12 V output. C19 and C20 capacitors reduce the radiation EMI noise.

RC snubber networks comprising R16 and C17 for Q2, R7 and C6 for Q1 damp high frequency ringing across SR FETs, which results from leakage inductance of the transformer windings and the secondary trace inductances.
The gates of Q1 and Q2 are turned on based on the winding voltage sensed via R4 and the FWD pin of the IC. In continuous conduction mode operation, the power MOSFET is turned off just prior to the secondary side controller commanding a new switching cycle from the primary. In discontinuous mode the MOSFET is turned off when the voltage drop across the MOSFET falls below a threshold (VSR(TH)). Secondary side control of the primary side MOSFET ensure that it is never on simultaneously with the synchronous rectification MOSFET. The MOSFET drive signal is output on the SR/P pin.

The secondary side of the IC is self-powered from either the secondary winding forward voltage or the output voltage. The output voltage powers the device, fed into the VO pin and charges the decoupling capacitor C7 via R4 and an internal regulator. The unit enters auto-restart when the sensed output voltage is lower than 3 V.

Resistor R8, R15 and R6 form a voltage divider network that senses the output voltage from both outputs for better cross-regulation. Zener diode VR2 improves the cross regulation when only the 5 V output is loaded, which results in the 12 V output operating at the higher end of the specification. The InnoSwitch-EP IC has an internal reference of 1.265 V. Feedback compensation networks comprising capacitors C15, C21 and resistors R14, R19 reduce the output ripple voltage. Capacitor C8 provides decoupling from high frequency noise affecting power supply operation. Total output current is sensed by R20 and R21 with a threshold of approximately 33 mV to reduce losses. Once the current sense threshold across these resistors is exceeded, the device adjusts the number of switch pulses to maintain a fixed output current.

Sources: Power Integrations

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