# 40V-50V Step-down DC-DC chopper

#### cloud623

Joined Dec 16, 2007
9
the output voltage cannot over 15V, but my aspect output voltage is over 40V, and anyone can help me solve this problem?

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#### thingmaker3

Joined May 16, 2005
5,084
An n-channel MOSFET must have the gate at a higher potential than the source in order to turn on. Use a p-channel MOSFET for your high-side swtich instead of an n-channel, or use a charge pump to drive your high-side n-channel MOSFET.

#### mik3

Joined Feb 4, 2008
4,843
the output voltage cannot over 15V, but my aspect output voltage is over 40V, and anyone can help me solve this problem?
This strange that your output can reach 15 volts!!!! It should reach about 12 volts, depending on your MOSFET.

Your maximum voltage at the MOSFET gate is 15 volts. If your output voltage is reaching 15 volts then Vgate-Vsource is getting less and less until this dfference goes below the threshold voltage of your MOSFET and it cant turn on more. So, your output voltage balances at a voltage less than 15 volts which is capable just to turn the MOSFET on.
To solve your problem you can remove the 15 volt source and connect it directly to 40 volts but be careful in any case not to exceed the maximum gate-source voltage and destroy the mosfet. Another solution is to use an IC called MOSFET gate driver to drive the gate of your MOSFET which solves this problem or use a p-channel MOSFET as thingmaker3 suggested.

#### thingmaker3

Joined May 16, 2005
5,084
Golly, I completely misread. I thought "aspect output" was a typo for "expected output" and that the OP was complaining about not being able to go above 15V o/p.

I guess I really should wait until after my second cup of coffee before posting a reply.

#### RmACK

Joined Nov 23, 2007
54
To expand on what mik3 said, an alternative to the expensive high side driver IC is pretty much your circuit, with a minor change. The transistor can easily be your high side driver for the N type mosfet.

You could achieve this, (as long as the 15V supply is isolated, i.e. it does not have a common negative/earth with the 60V supply see below how to test) by simply connecting the negative of the 15V supply to the source of the mosfet i.e. the +ve variable output instead of the -ve. This would give a gate-source voltage of 15V whenever the opto is turned on hence turning on the mosfet.

A way to test if they are isolated: disconnect 15 volt supply and measure the resistance from it's negative to the negative of the 60V supply (with them off). Should be open circuit. Now connect a 10k ohm resistor between the negatives (this should be the only thing connected to the 15 volt supply). Power both supplies up & measure the voltage from negative of one supply to the other, ie. across the resistor. Should be zero. If so then you can use the above idea!

With regard to the actual output voltage: that is controlled by the duty cycle of your pwm which you have not showed the origin of. Presumeably you have a feedback network into it.

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#### thingmaker3

Joined May 16, 2005
5,084
"Expensive" high-side driver IC? You can get 'em for little over a buck each...

#### RmACK

Joined Nov 23, 2007
54

#### thingmaker3

Joined May 16, 2005
5,084
It may indeed be what they were after, but bootstrapping would require a little more than merely shifting over the negative of the 15V supply. Most of the components would have to be re-arranged.

I'll play around with the circuit this weekend to see if I find inspiration.

#### RmACK

Joined Nov 23, 2007
54
If your 15V supply is isolated and allowed to float such that it's -ve is at the same potential as the source of the mosfet, you have your extra voltage above the 60 inherently.

Sometimes this is done instead of bootstrapping with a cap, e.g. if you want to allow very high duty cycles (>95% is a common rule of thumb for bootstrapping being unwise) It just seemed like the simplest option if his supply is isolated.

#### thingmaker3

Joined May 16, 2005
5,084
This might work, but only with isolated supplies:

#### RmACK

Joined Nov 23, 2007
54
Yes that jpg is exactly what I think the original designer intended to do