EPR-32 Datasheet, PDF (8/36 Pages) Power Integrations, Inc. – Engineering Prototype Report for EP-32 − TOPSwitch-GX 25 W Multiple Output DVD, Set-top Box Power Supply Using TOP245P

EPR-32 - 25 W Multiple Output TOP245P Supply
13-Jul-06
to maintain regulation as the load is further reduced. This greatly reduces switching
losses, maintaining high standby efficiency and low no-load power consumption.
Current mode control above 50% duty cycle requires slope compensation and this is
provided by a ramp signal generated from the bias winding via R2 and C16. Capacitor
C16 also serves as a high frequency roll-off filter. The gain/phase results presented
(Section 12) show excellent margin under all operating conditions.
4.5 Secondary 3.3 V and 5 V Shunt Regulator
To meet the cross-regulation requirement of the 3.3 V output at maximum load while the
5 V output is at minimum load or vice versa, a low cost secondary side shunt regulator
was added between the 3.3 V and 5 V outputs. This is formed by R14, R13, Q5, R16,
R15, Q4, Q1, D8, and D9. This provides current from the 5 V output into the 3.3 V output
when the voltage difference between the two outputs becomes unacceptable. This
threshold is set by R14, R15 and Q5, which in turn drive Q4 and Q1, providing current to
the 3.3 V output via D8 and D9.
Unlike a linear regulator, this circuit dissipates very little power and the addition of D8 and
D9 reduces the dissipation in Q1 such that no heatsink is required at all. The circuit
configuration provides enough temperature compensation to meet the 0 °C to 50 °C
ambient temperature specification.
In applications where the 3.3 V and 5 V loads track (the minimum and maximum loads on
both outputs occur at the same time) then this circuit may be removed. Note that this
circuit is not needed to meet no-load regulation.
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