EPR-32 datasheet(7/36 Pages) POWERINT

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13-Jul-06

EPR-32 - 25 W Multiple Output TOP245P Supply

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drain ringing. The slow recovery time, compared to fast or ultra-fast diodes, allows

recovery of some of the clamp energy, improving efficiency.

4.3

Output Rectification

The secondary of the transformer is rectified and filtered by D7, C15, D11, C11, D10,

C12, D12, and C9. For better voltage centering and regulation, the 12 V output is DC

stacked on top of the 5 V output; the start of the 12 V winding is connected to the cathode

of D11. A small ferrite bead, L5, was used to center the output and improve cross-

regulation by reducing the effect of secondary leakage inductance.

Post-filters (L2, C20, L3, C17, L4, C16, R8, and C6) remove switching noise and further

reduce switching ripple.

4.4

Output Feedback and Control

DC feedback to the output voltage regulator error amp (U3) comes from a combination of

the 3.3 V output, via R12, and 5 V output, via R11. Together with resistor R9, these form

a resistor divider, the center point that is tied to the 2.5 V REF pin of U3. Capacitor C14

and R10 roll off the high frequency gain of U3 while R17 sets the overall DC gain.

In a TOPSwitch-GX design, regulation of the output is normally provided by voltage mode

PWM control. The current into the CONTROL pin sets the duty cycle of the internal

MOSFET. The duty cycle control operates over a CONTROL pin current of 2 mA to 6

mA. Current below this level is used to supply power to the IC.

In this design control is accomplished by employing the externally programmable current

limit function of the TOPSwitch-GX family, in this case via the M pin. This implements

current mode control rather than using voltage mode PWM control via the CONTROL pin.

The first ~2 mA of feedback current is fed into the CONTROL pin. This provides the

supply current for operation, but leaves the duty cycle at the internal device maximum.

Feedback current above ~2 mA forward biases Q3 and pulls up R6 via R4.

The

characteristic of the M pin is such that increasing sink current (current out of the pin)

increases the primary current limit. Therefore, as the feedback current increases, the sink

current decreases and the primary current limit reduces, thereby allowing the output

voltage feedback loop to control the primary peak current. Resistor R6 sets the peak

current limit (startup and overload) and R4 ensures that the maximum source current

(current into the M pin) stays below 44

µA to prevent the device from operating in the line

sensing mode of the M pin. As any current above 2 mA engages the M pin control, the

current into the CONTROL pin is limited to this level and therefore, the PWM function of

the CONTROL pin does not determine the duty cycle.

As the load is reduced, the primary current limit reduces until the remote ON/OFF (inhibit)

threshold is reached at an M pin sink current of approximately 27

µA. The supply then

operates with a fixed 25% current limit, but at a reduced and variable switching frequency,

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