What's wrong with this circuit ?? - Page 1
Forum section: electronics / beginners

author: yashrk
author_rank: Frequent Contributor
timestamp: February 21, 2015, 02:19:35 am
content: Hello guys,I was building an h bridge motor driver circuit. Then I found this circuit on talking electronics website which looked like similar to what I was thinking of building, I simulated the design and it worked. But I found that on website it says it's a faulty circuit but I couldn't understand what's wrong with the circuit. Please help me to understand what's wrong with it and what I can do if I have to drive this circuit in the same configuration.Thank you,Yash.

author: kingofkya
author_rank: Regular Contributor
timestamp: February 21, 2015, 02:46:48 am
content: You might want to read up on pchannel vs nchannel fets:)

author: yashrk
author_rank: Frequent Contributor
timestamp: February 21, 2015, 02:58:40 am
content: How could I solve the problem. Will using p channel mosfet will it work

author: yashrk
author_rank: Frequent Contributor
timestamp: February 21, 2015, 03:28:15 am
content: Sorry my bad I didn't notice that both gate n source are on same potential when turned on. That the problem right?

author: TerminalJack505
author_rank: Super Contributor
timestamp: February 21, 2015, 03:36:35 am
content: Yep, I think you found the issue.If you want to use all N-channel MOSFETs then you will need to drive the gates with a voltage that is higher than 12V. This will ensure that the top two MOSFETs arefullyon.

author: yashrk
author_rank: Frequent Contributor
timestamp: February 21, 2015, 03:55:02 am
content: So what do you suggest should I do to dive this circuit, I am using an arduino to drive this circuit and should I use diode and capacitor bootstrap circuit or there any better option

author: Psi
author_rank: Super Contributor
timestamp: February 21, 2015, 03:57:36 am
content: You can buy high side drivers that use a N channel mosfet and an internal boost converter to solve the problem.They also have overcurrent/temp protection and a few other useful things and are not that much more expensive.Package is the same too, dpak/to220 style. but usually with a few more pins.A 60V 60A 16mR P-channel fet i was using for a project a while back cost ~$3 and one of these chips cost ~$4

author: TerminalJack505
author_rank: Super Contributor
timestamp: February 21, 2015, 04:20:51 am
content: You can also use P-channel MOSFETs for the two top transistors. It might make things simpler.

author: yashrk
author_rank: Frequent Contributor
timestamp: February 21, 2015, 04:27:24 am
content: I will try to get higher voltage on gate keeping the same configuration, if the problem still rises I will switch to p and n channel configuration.Guys thank you for your response. You all are always very helpful.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 21, 2015, 06:58:41 am
content: Note that the complementary bridge willshort out the power supply during the time the gate voltage is transitioning. If rise/fall time is quick (using a gate driver chip is a good idea), this can be handled with some supply inductance and damping (no, you DON'T want as low a supply impedance as possible!). If transition time is slow (some transistors and resistors?), you'll have little choice but to use series resistance in the supply, which will impair efficiency driving whatever the load is.The all-N-channel case is perfectly acceptable, as long as your "high side" gate drive voltage is above the supply. This is usually provided by a bootstrap gate driver, like HIP2101.The gates can also be driven together as shown, but you are limited on supply voltage: in order to fully turn on the high side transistors, you need Vgs at least 5V, which means Vg will be 5V above the supply. But the low side transistor will have Vgs(max) rated for 20 or 30V, so your supply can't be more than 25 or 30V, worst case. For this reason, such a configuration is useful only rarely, and at that, usually as a cost-cutting measure.Tim

author: yashrk
author_rank: Frequent Contributor
timestamp: February 21, 2015, 07:26:28 am
content: Thank you @T3sl4co1l