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Power supply 800V -> 48V using INN3996CQ

Posted by: AlbertoFerrero on 10/07/2021


I would like to design a power supply from 200-800Vdc to 48Vdc/15W using a INN3996CQ.

My approach is to get inspiration from the P.I. application "DER-889q" for a power supply with output 12V/15W. In that application the pin 6 (Vout) is directly connected to the output to supply the secondary side of the chip, but I can't do the same because 48V is to high (maximum rating for Vout is 27V). So my questions are:

- can I use a linear regulator (a zener, a zener+BJT, a LDO, ...) to the pin Vout to supply the secondary side from 48V?
- what is the current absorption (I didn't find this information on the datasheet)?
- If I renounce to the synchronous rectifier can I reduce the current consumption of the secondary side?

Many thanks in advance for your support,



Submitted by PI-Snorlax on 10/14/2021

Hi Alberto,

For questions 1 and 2:

Based from the block diagram of InnoSwitch3-AQ, Vout pin is used to provide current to the secondary IC.

The 4.4V Regulator block is supplied either by Vout or the FWD pin.

Since your target output voltage is 48V (which is way above the pins 27V rating), you can consider the following:

1. Using a zener diode to protect the Vout Pin from overvoltage.

DER-666 (30V, 45W Output using InnoSwitch3CE) implemented this type of solution.

DER-666 was able to achieve high efficiency (~90%) amidst the implementation of zener diode since zener diode was just clamping approximately 9V.

For your case, zener diode will need to clamp approximately 27V to keep the Vout pin at a safe level which could have an impact on your overall efficiency.


2. Aside from using zener diode, another approach is to implement an “Auxiliary Winding” which is wound in the same direction as the 48V output secondary winding.

This “Auxiliary Winding” should be designed (with smaller turns ratio) such that the voltage across this winding (at 48V output of the secondary winding) would not exceed Vout Pin rating.

Then you can now connect the Vout Pin as well as the FWD pin to this “Auxiliary Winding”.

You can use DER-646 as your reference for this approach.


3. Another possibility (though no related DERs as of the moment) is to implement a center tapped transformer design.

4. For the required current to be supplied to Vout pin, you can refer to DER-717 where a resistor is used to limit the current through the Vout pin.

This resistor was selected to ensure enough current will be supplied to the 4.4V Regulator block.

Using same resistor value at a higher output voltage (for your case 48V) would still ensure proper operation of the 4.4V Regulator block.



For question 3:

If you are referring to the current consumption of the INN3996CQ’s secondary controller, this is just minimal even with Synchronous Rectification enabled.

In typical applications, the benefit in efficiency when using Synchronous Rectification is more pronounced  than the gain on secondary side current consumption with SR disabled.

However for your target application, since your target Vout is around 48V and depending on your transformer turns ratio, Vds rating for the SR Fet might be high and it will be difficult for you to find SR Fet with high Vds rating with Vgs of 4V.

INN3996CQ’s SR Pin Drive voltage is typically around 4V thus you might end up disabling this function.