PSU overvolting when braking large h-bridge/Motor - Page 1
Forum section: electronics / beginners

author: danieldk
author_rank: Newbie
timestamp: October 08, 2013, 10:15:50 am
content: Hi GuysMy name is Daniel - and this is my first post on EEVblog.I'm somewhat a novice when it comes to designing and troubleshooting. allthough i have done a lot of tinkering.I'm glad i found this site it seems like a great place!My current project is a large actuator which i'll need later on another project... The actuater is rated at 1kw@48v. stall current is much higherI'm currently driving it with an OSMC h-bridgehttp://www.robotpower.com/downloads/OSMC_project_documentation_V4_25.pdfThe OSMC is hooked up to a 3kw/48v switchmode PSU and both are driving the actuator just fine even at motor smoking stall currentsMy troubles begin when i need to brake a heavy load. The voltage on the output side of the psu shoots up to more than 60 volts and the psu goes into protect mode.i was kinda expecting this behavior, but i cant get my head around solving it.so far i've tried:Changeing the way i drive the h-bridge - but without some very accurate feedback from the motor - this seems like a dead end.i've added a 100,000UF cap to the psu output. - this did mitigate it somewhat but braking more than a few seconds still drives the voltage up..Idealy i would like to dump the excess current/voltage into a braking resistor. However i don't know where to start in designing a circuit capable of detecting and dumping the excess voltage/current..Hope you can help me out.Thanks!CheersDaniel

author: Zero999
author_rank: Super Contributor
timestamp: October 08, 2013, 01:00:39 pm
content: Try the TL431.See the circuit in figure 24 on page 29. Set the voltage to above the maximum nominal voltage but below the voltage which will trigger the PSU into protection mode. You could put a resistor in series with the transistor to help it do dissipate the power and will probably need to use a Darlington/Sziklai pair for more gain.http://www.ti.com/lit/ds/symlink/tl431.pdf

author: danieldk
author_rank: Newbie
timestamp: November 18, 2013, 01:19:54 pm
content: I didn't have a tl431 but i did have a comparator lm339...The good:the above circuit is keeping voltages in check - voltages reaches 50v and stays there!The bad:The hexfet is getting really hot after 2 seconds of braking. I suspect it is because i'm not driving it into saturation quickly enough...I'm receiving a bunch of tl431 tomorrow, and i will try the circuit frm the datasheet...any thoughts is very welcome!CheersDaniel

author: Zero999
author_rank: Super Contributor
timestamp: November 18, 2013, 04:52:05 pm
content: The MOSFET will get hot because it's dissipating the voltage.The comparator will probably oscillate as it enters the linear region and there are a couple of pull-up resistors missing.

author: Marco
author_rank: Super Contributor
timestamp: November 18, 2013, 05:53:07 pm
content: You're never actually turning the MOSFET off ... it has to turn off through the collector base leakage current.Put the energy dump resistor on the drain of the MOSFET, replace the PNP transistor with a proper MOSFET driver (at least use a totem pole and a pull up resistor on the comparator output) and add some feedback on the comparator for hysteresis.

author: danieldk
author_rank: Newbie
timestamp: November 19, 2013, 12:02:50 am
content: scoping the collector of the bc557 shows a nice turn on characteristic. however the turn off oscillates 50 or so times...

author: Andy Watson
author_rank: Super Contributor
timestamp: November 19, 2013, 02:51:12 am
content: I guess 330k is better than an open circuit - if you don't mind waiting a week for it switch off.Check the drain-gate capacitance on the data sheet, factor-in a 10-100 fold increase due to Miller feedback and use this value to calculate a RC time constant that is significantly faster than the required switching speed. I anticipate you will find this gives a stupidly low value of R - there's is a reason that active MOSFET drivers are popular.I think the oscillation is to be expected - you need to reduce the control loop gain (at the moment you are effectively switching from closed to open circuit - that's a really high gain!). You can't magic the braking energy away, it will need to be dissipated somewhere. Marco has already suggested the resistor in the drain - this will also have the effect of reducing the loop gain of the control circuit.

author: danieldk
author_rank: Newbie
timestamp: November 19, 2013, 08:01:57 am
content: Hi AndyThank you for the explanation -With my new found knowledge, a totem pole, like marco said, seems like a good idea... Do you guys think that will be sufficient for this project?Are you aware of a mosfet driver which is used often?i will find a toaster or something to dump the load intoThanks AgainCheersDaniel

author: poorchava
author_rank: Super Contributor
timestamp: November 19, 2013, 08:24:55 am
content: You simply need to have a push-pull output stage. With smaller loads it's also a viable solution to add a few large capacitors. The bigger the capacitance, the smaller the voltage increase will be, but at those power levels that you have there, it doesn't seem feasible.

author: danieldk
author_rank: Newbie
timestamp: November 20, 2013, 12:34:39 am
content: So i used a push-pull stage to drive the irf1405. I used a 1k resistor -> +12v to pull the common base high on the pnp/npn transistor pair.heres the output from the push-pull stage during a small brake-command:The inertia of the motor alone is still enough to burn the irf1405 - that can't be more than 10-50watts of power being generated...Allthough i havent added a power-resistor or hysteresis..i'm thinking; that small amount of power should easily be dissipated in the wires and windings of the motor....from the, albeit poor, snapshot on the scope, it seems i really need that hysteresis and i need to drive the mosfet harder still as i'm only seeing 8 volts?would you recommend i switch to using the TL431 instead of the lm339?ZzzZz its getting late - cyathanks!Daniel

author: amyk
author_rank: Super Contributor
timestamp: November 20, 2013, 08:25:55 am
content: That MOSFET is designed to operate up to 175C so it might be perfectly fine even if it's getting far too hot to touch (If it's desoldering itself then it's probably getting a bit too hot...).How about a high-current relay?