# MOSFETs connected in series forming an H-bridge circuit for switching

I want to develop an H-bridge circuit using 2 MOSFET switches connected such that when the switch 1 is on (switch 2 off) the output voltage should be +5V and when the switch 2 on (switch 1 off) the output should be -5 V. Can anyone tell me which type of MOSFETs (N channel or P channel) I should use and also, which terminal should be connected to the Vdd (drain or source) and which terminals of the MOSFETs should be connected together/series? I will be using 2 input high side driver to control their gate voltages.

• Have you googled H bridges using mosfets and tried to understand any circuits - if you have then you ought to provide a link. Jul 18 '14 at 19:06

The following circuit should work for your purpose:

From left to right, we have a 25Hz square wave source. The actual frequency doesn't really matter, but what matters is that it is a 10V source, to fully turn the MOSFET on. Anything lower and you will start limiting current, so the voltage across your load will decrease (V = IR for Ohmic devices). Next, we see two 5V voltage sources in series with a center ground. If you probe (relative to ground) either of the other two terminals, you will see either +5V or -5V. Next, the +5V terminal is connected to the drain of an N-channel MOSFET. The -5V terminal is connected to the drain of a P-channel MOSFET, and the sources are tied together (I believe CMOS style?). The output of the source is shown in the scope window at the bottom. The yellow line (actually showing current) is zero, so it swings from +5V to -5V at 25Hz.

If you use this, make sure you understand MOSFET activation regions well enough to drive them at the correct voltage for their intended use. Also make sure to connect the load to the common source on the MOSFETs and to ground, not to the -5V terminal. Ground is a center "tap" (not actually, since it isn't a transformer coil), so be sure to use that as 0V/Gnd for any loads connected to this circuit. It's also worth mentioning that using a CMOS setup means you don't need to use an inverter, or even separate inputs to the MOSFETS. You only need to make sure your driving circuit runs a high enough voltage, and can provide enough current to overcome gate capacitance on two MOSFETs.

I hope that helps!

What you are looking for, I believe, is referred to as a "Half Bridge". The circuit looks like this:

This setup is often found in solid state power supplies and Tesla coils. Looking at the symbols, you can see that it uses two N-channel enhancement-mode MOSFETs and two capacitors. When Q1 is on, current flows from Vcc through Q1, through the load, through C1, to ground. When Q2 is on, C1 is discharged through the load (causing current to flow in the opposite direction).

Hope this helps.

EDIT: I should have mentioned before that this circuit requires a MOSFET gate driver and a gate drive isolation transformer. I would recommend either a UCC27425 (4-Amp dual, one inverting and one non-inverting) or a combination of the TC4420 (non-inverting) and the TC4429 (inverting) drivers. Obviously you will need to tailor this to the amount of current your load will draw. Connect the driver to a gate drive transformer with two secondaries wound for opposite phases, and connect the secondaries to the FETs. I have edited the above image to reflect this change. There are higher-power versions of these chips that you may want to look into.

• Then you have the problem of how to switch Q1. Jul 18 '14 at 20:37
• Using an inverter on one of the FET inputs should work, assuming you are using the same signal to control both of the FETs Jul 18 '14 at 20:42
• If the idea is to connect the left side of the load (and thus, Q1's source) to Vcc, then you'd need to apply at Q1's gate a voltage higher than Vcc. How do you accomplish that with an inverter? Jul 18 '14 at 20:48
• Presumably this is for an AC only load? Also using an inverter won't work because the upper FET is a source follower. Jul 18 '14 at 22:16
• yep, sorry--that was my mistake. I'd recommend using a gate driver chip, perhaps the UCC27425 from TI. It is a dual driver, one inverting and one non-inverting, which is perfect for driving a half bridge Jul 19 '14 at 4:39