Half Bridge Inverter for DC-AC Conversion - Page 1
Forum section: electronics / projects

author: bentx0rx0
author_rank: Contributor
timestamp: March 13, 2014, 02:56:05 am
content: I'm looking at building a half bridge inverter for converting 20 volts DC to the equivalent AC signal. I haven't really been able to find very much literature on Half Bridge Inverters however. Almost everything I find lends to a design that looks like this:How long should S1 and S2 turn on for? Do I need an MCU to control those MOSFETS?I see that I need diodes to preserve continuity for inductive loads, but is there anything else I should be looking out for? I've got it feeding an impedance matching circuit, but I can't say I have much experience with DC to AC conversion. Thank You!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 13, 2014, 03:25:02 am
content: Those... IGBT symbols aren't right? The not-going-anywhere diagonal line is supposed to be pointed up...?! And the substrate is supposed to be segmented like an enhancement mode MOSFET's. Weird...Anyways...If you're using MOSFETs, the diodes come for free as parasitic body diodes in the devices (however, you may still want external diodes, to reduce the voltage drop or improve performance at high speeds >> 200kHz). If you're using IGBTs (as I assume the schematic is implying), check that you choose devices with "co-pack" diodes; an IGBT, like a BJT, does not intrinsically handle reverse voltage (and can be destroyed by it without protection!). If you're using BJTs, reverse bias isn't necessarily destructive, and kind of depends on the driver, but probably will be anyway, so should have diodes too.As for timing, as long as S1 turns off before S2 turns on (with a "dead time" of, say, 100ns or thereabouts), you'll be fine.It's worth noting that there are many situations where you *do* want them both on at the same time -- but, this requires series inductance in the supply, and snubbers on the transistors; you can't just have bypass caps right by the transistors for that to work well. (These situations don't involve inductive loads, so you should be okay with that.)What you want to generate the signals depends on what you want it for. AC just for pissing around? Who the hell cares. Use a 555 (not quite, you need high side drive -- a matter for another thread, but suffice it to say, it's important). Regulated DC output, maybe through a transformer? Consider a TL494 (or KA7500, or SG3524, or..) switchmode controller. Motor drive (low frequency AC, VFD) output? Something else, perhaps a microcontroller indeed (but, you'll want to be sure of what you're doing before committing to a circuit, lest the magic smoke get released). Class D audio amplifer? There's chips for that (though I don't have numbers offhand).Tim

author: nickm
author_rank: Regular Contributor
timestamp: March 13, 2014, 03:30:21 am
content: Research Class D output stages, they are half-bridges. There is a lot of information strewn across the internet about them. I suggest you start by looking for a half bridge gate driver because their datasheets and app notes will contain a lot of information that theorists leave out. International Rectifier is a popular manufacturer of gate drivers.

author: johansen
author_rank: Super Contributor
timestamp: March 13, 2014, 04:27:59 am
content: keep to a minimum the inductance of those capacitors, and the transistors, and make sure the capacitors can handle the ripple current.

author: bentx0rx0
author_rank: Contributor
timestamp: March 14, 2014, 06:34:48 am
content: Are there any specific formulas for the values of the two capacitors? I see the voltage across them, but I'm not sure if that directly translates into a value

author: johansen
author_rank: Super Contributor
timestamp: March 14, 2014, 06:55:54 am
content: the uF value will set the voltage ripple at that middle node, which should be kept to a minimum.how much current do you expect to pull and at what frequency?

author: bentx0rx0
author_rank: Contributor
timestamp: March 15, 2014, 05:46:41 am
content: I expect to pull 4A at a frequency of 200kHz. VDC is 20V. I can simulate it, but I'd appreciate it if you could explain how you came to the values

author: johansen
author_rank: Super Contributor
timestamp: March 15, 2014, 10:44:19 am
content: 20v/4A is 5 ohms.so say you want less than 1 volt of ripple on that node, so you need an impedance of .25 ohms.works out to about 4uF, or 2uF each.Xc=1/(2*pi*F*C)

author: diyaudio
author_rank: Frequent Contributor
timestamp: March 15, 2014, 04:39:30 pm
content: Quote from: bentx0rx0 on March 13, 2014, 02:56:05 amI don't see any magnetics most of your challenge will be in this area, seehttp://www.diysmps.com/forums/content.phpthe guys have successfully implemented what you described.

author: bentx0rx0
author_rank: Contributor
timestamp: March 15, 2014, 06:41:01 pm
content: I started running some simulations based on the provided values. Using 2uF capacitors and 2n7002 I got these results:Circuit Setup in MultiSim:I think I'm getting the right waveform, but it's negative, and the output voltage is kinda iffy

author: johansen
author_rank: Super Contributor
timestamp: March 15, 2014, 07:07:14 pm
content: lol yeah that is all wrong.the gate drive source needs to float relative to the igbt, not the other igbt, and they need to be synchronized, as it is now you are turning on both fets at the same time.

author: bentx0rx0
author_rank: Contributor
timestamp: March 15, 2014, 07:59:11 pm
content: Yes it is. I think I've almost got it. Here's the new oscilloscope results:and the new circuit:The only thing that bothers me know is that the voltage is negative, and that would suggest the polarity is the inverse of the diagram. However I've found other diagrams with the polarity switched, so I think it's just the diagram.With the oscilloscope connections inverted:Thanks for the help everyone. I guess the first circuit wasn't working because the 2n7002 is not a logic level mosfet. If I've read this correctly, a non logic level mosfet needs a higher voltage on it's gate than it's source, while logic level mosfets have a charge pump within that negates that need.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 15, 2014, 11:45:12 pm
content: Singular matrix:Because there is no DC path to the capacitor divider node, the voltage on that node is undefined. If you're lucky, the default RSHUNT setting (see Interactive Simulation Settings dialog) will initialize the node to 0V, thus the "scope" will measure a square wave of 0 to 20V, not -10 to 10.These settings are better than default, particularly for switching circuits:http://seventransistorlabs.com/Images/SimSettings1.pnghttp://seventransistorlabs.com/Images/SimSettings2.pnghttp://seventransistorlabs.com/Images/SimSettings3.pngNote also the "scope" is not a scope to the circuit (it has infinite impedance, whereas a real probe has resistance and capacitance), so the capacitor will not charge through it either.Tim

author: I_may_be_drunk_right_now
author_rank: Contributor
timestamp: March 16, 2014, 01:31:17 am
content: Microchip AN1114 & AN1207http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1824&appnote=en532115http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1824&appnote=en536097You'll enjoy AN1207 p39.

author: bentx0rx0
author_rank: Contributor
timestamp: March 16, 2014, 02:02:10 am
content: You may be drunk right now, but that is a very helpful set of datasheets. The section on half bridge inverter and the associated formulas are AWESOME!T3sl4co1l, those settings helped a lot. The oscilloscope now outputs -10 to 10V. Is it accurate to use these settings for all future values

author: I_may_be_drunk_right_now
author_rank: Contributor
timestamp: March 16, 2014, 02:39:28 am
content: Quote from: bentx0rx0 on March 16, 2014, 02:02:10 amBOOYAH! Glad I could help. I pour myself a drink every time I help. Even when I don't, really.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 16, 2014, 02:53:04 am
content: Those settings will work for many things but always remember SPICE is a computational model, NOT a representation of the real world, never has never will.I've made many circuits which bunged up the simulator; often, changing one component value in the circuit will make it work or fail; and a tweak of the settings may be similarly chaotic.When you get one of these (most often "timestep too small"), the Convergence Assistant is mostly useless; if you watch what it does, you can remember those steps and try them yourself in as much time, but half the time it succeeds by breaking the circuit: for instance, cranking up the RELTOL (so it no longer analyzes small errors) and initial timestep, which will break an oscillator (it never starts up, or actually rings down and stops completely because the oscillation amplitude just looks like error to ignore!).The model is just a model. Never expect it to understand what you think you're doing. Creating a useful, representative model will always be the user's responsibility.Tim