Buck Converter using mosfet with TLP250 problem

Thread Starter


Joined Oct 21, 2017
Hello friends,

I am trying to design a buck converter that will have an input voltage of 50V output at 25V. I applied 50Khz 50% duty cycle PWM signal but I obtained values:
TLP 250 voltage supply and input voltage supply are different voltage sources

Load= 39 ohms
TLP250 voltage =12.4 V
TLP250 current= 1.54 A
Input voltage =20.4V
Input current= 0.11 A
Output voltage rating=8.45

Normally the input current value should be at least: 8.45 / 39=0.22 A
What is the problem my circuit? If you help me, I will be happy


Joined Mar 14, 2008
Should my circuit be this?
Remember you trying to drive the gate-source voltage (not gate to ground) of Q1 to a plus 10V to turn it on.

Remove the U4-5 connection to IN-.
Connect U4-8 to the positive of a separate (isolated) 10V supply.
Connect the negative of the supply and U4-5 to Q1's source terminal.


Joined Aug 23, 2012
For that to work the TPL250 supply voltage must be floating with respect to ground, with the supply ground connected to U4 V-.
That won't work to fully turn on the N-MOSFET.
Thanks for remind me that.
When I saw the circuit then I thought that the TS should be prepared ok but just wired error, until I saw you reminded me the error, I think the Vgs is not enough, I searched the charge pump that you posted, but I'm not sure is it suit for this application or not, and I may using voltage doubler and voltage divider to provides the Vgs.


Joined Jan 7, 2018
I may agree with you, and if somebody is offended I am sorry, but I was looking for a circuit similar with this (my ic is tlp350) and google provided me this link yesterday. Since my comment sudgest another solution that may help others to find a way to drive their mosfet I considered it a good fit. I am sure that original poster found a way to make the circuit work, but in the end we are a community that share knowledge and tries to help each-other.