High frequency mains switching - Page 1
Forum section: electronics / projects

author: Phaedrus
author_rank: Frequent Contributor
timestamp: May 30, 2014, 11:12:01 pm
content: This is a project I've discussed on here before, but had to take some time away from it. Basically, I need a way to switch mains supply to a high-wattage SMPS at an arbitrary AC phase angle and voltage. The sensing and logic is easy. What's given me trouble is the AC switching. Mosfets are not suitable because of the body diode blocking in one direction when turned on. Relays are too slow. SSRs are too expensive. After some research, I've settled on IGBTs.However, I haven't worked with them much, so I want to make sure I'm driving it correctly. Isolation is a big concern here; if there's a short between the IGBT's C or E to G, I've got the optocoupler (not chosen yet) to protect the digital circuitry, and a 1N4006 to protect the power supply.See attached schematicEDIT: Whoops, wrong part # on the Q2 PNP. Should be ZXTP2040F

author: dannyf
author_rank: Super Contributor
timestamp: May 31, 2014, 12:03:52 am
content: QuoteWatch the current through the phototransistor.

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 31, 2014, 12:04:38 am
content: You want to use a triac or SCR. Drive that with a special optoisolator or a pulse transformer.

author: retrolefty
author_rank: Super Contributor
timestamp: May 31, 2014, 12:17:13 am
content: Code:See that arrow symbol on the IGBT's emitter lead ? Still think it can conduct AC current? You need to look at higher power Thyristor devices (SCR, Traic).

author: digsys
author_rank: Unknown
timestamp: May 31, 2014, 12:33:48 am
content: Quote from: PhaedrusNot true. A VERY common trick is to gang 2 FETs in series, diodes facing each other. You tun BOTH on/off at the same time. FETs don't reallyhave a polarity. There are bridge chips that do this (simplifies the design a bit), but for higher voltages you'd need to float the drive.

author: SeanB
author_rank: Super Contributor
timestamp: May 31, 2014, 05:49:42 am
content: If you want to use the IGBT use a bridge rectifier and use the IGBT to short the output DC side, and have it in series. Your driver will have issues, you will need a pull down or better yet a logic level output opto so the totem pole driver can work.

author: Phaedrus
author_rank: Frequent Contributor
timestamp: May 31, 2014, 02:40:17 pm
content: Quote from: digsys on May 31, 2014, 12:33:48 amI've tried that, but it never works in my SPICE simulations. Oftentimes I seem to still get positive voltage blocked, but negative voltage passes unattenuated.Triacs don't work for me because they latch on. SCRs don't work for me because they take a half-cycle of reverse polarity to turn off. I don't care about how voltage is switched on: I care about how it is switched off. I need <1ms fall time, on demand.

author: Phaedrus
author_rank: Frequent Contributor
timestamp: May 31, 2014, 02:44:59 pm
content: They only solution I've found that mostly works is a mosfet or igbt behind a bridge rectifier. This works, but it's ugly and the diode voltage drop would distort the waveform.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 31, 2014, 03:49:13 pm
content: Quote from: Phaedrus on May 31, 2014, 02:40:17 pmYou probably wired it wrong. The drive must be referenced to the common source node:Same is true of the IGBT; your above example is a smoke factory. IGBTs don't appreciate reverse voltage, so much so that they frequently go to the length of adding an antiseries diode in the package (a co-pack, two dies in one package), so it can be used transparently as a MOSFET drop-in.Tim

author: Zero999
author_rank: Super Contributor
timestamp: May 31, 2014, 04:33:19 pm
content: That circuit won't work. When the mains voltage is positive, it will act as an emitter follower and when the mains voltage is negative, it'll blow the IGBT.

author: Panduit
author_rank: Newbie
timestamp: June 01, 2014, 01:08:11 pm
content: TLP250http://media.digikey.com/pdf/Data%20Sheets/Toshiba%20PDFs/TLP250.pdfBridge Rectifier 50ASorry , my bad english.

author: dannyf
author_rank: Super Contributor
timestamp: June 01, 2014, 02:11:30 pm
content: A DC switching device can be used to switch AC, with the help of a bridge rectifier.

author: max666
author_rank: Frequent Contributor
timestamp: June 01, 2014, 04:18:05 pm
content: How is the 1N4006 diode protecting your power supply? I can see it blocking the positive half of the mains voltage, but what about the negative?

author: Phaedrus
author_rank: Frequent Contributor
timestamp: June 05, 2014, 06:47:09 pm
content: Quote from: T3sl4co1l on May 31, 2014, 03:49:13 pmI'm trying to figure out how to make that work with a totem pole driver. It's the negative end of the totem pole referenced to the common source, correct? But does the power supply need to be referenced to the common source as well?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 06, 2014, 03:42:13 am
content: Quote from: Phaedrus on June 05, 2014, 06:47:09 pmIt's all there: the switch is your totem pole driver, which chooses between +V or source. The supply is referenced to source.You must have isolated power and switching if your control circuit cannot be connected to the line.Tim

author: Phaedrus
author_rank: Frequent Contributor
timestamp: June 06, 2014, 04:15:43 pm
content: Got it. I presume a typical transformer->bridge rectifier->7812 setup would work fine, so long as the transformer's secondary is referenced to the common source?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 06, 2014, 11:07:59 pm
content: Rpd1 and 2 are to the wrong ground, and you only need one driver since the gates only ever need to do the same thing.Why are TLP250 pins 6 and 7 labeled as VO+ and VO- respectively? They're both VO.Tim

author: Phaedrus
author_rank: Frequent Contributor
timestamp: June 06, 2014, 11:37:58 pm
content: The pulldowns need to be referenced to DrGND as well?They're both VO, don't know why I marked them +/-

author: Phaedrus
author_rank: Frequent Contributor
timestamp: June 06, 2014, 11:55:21 pm
content: I'm looking at a number of them, but there are some pretty good fets out there... IXFK160N30T is pretty impressive, 300V, 160A, 1400W, with 19mR RDSon, for $13/ea on Digikey. Maximum load for this system would be 1500W continuous, 1600-1800W peak (<10ms). Switching losses are not a huge concern, since this is acting mainly as a relay. So it will be normally on, then switch off. The digital circuit will then sent a trigger signal to my scope to make the measurements I need. The only reason I'm not using a relay is because I need <1ms switching speed, and it needs to be consistent on disconnect: relay arcing would cause too much inaccuracy for my needs.As for cooling, the company I work for makes something like 20% of the IT industry's heatsinks, globally.Easiest solution would just be a high-end CPU heatsink, we have some rated for 300W TDP, which can handle ~200W continuous dissipation. If I put the fets under an Intel CPU socket footprint I should be able to mount the heatsink no issue. Add in some generous copper fill and quality thermal grease and she should be right. My only concern would be low surface area for contact with the heatsink. I'll verify the temperature with a thermocouple before I put it into use.EDIT: Heck, I have some screwmount extruded aluminum heatsinks from some old PSUs I took apart. I could make a mosfet sandwich if I needed to.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: June 07, 2014, 02:36:19 am
content: Hi Phaedrus and the group,Here is LTspice model of the AC switch. In fact there are four variations in this model. I have posted this to promote thinking about the transients that are generated when switching an inductive resistive load. Even if you load is resistive, there will be some inductance from the line.I am not going to spoil the fun by posting the results.I have attached a zip file containing the model.Regards,Jay_Diddy_B

author: Phaedrus
author_rank: Frequent Contributor
timestamp: June 07, 2014, 03:01:50 am
content: The load would be various high-power SMPS, which have thorough EMI and surge protection built in, usually at least a MOV. Active PFC, so it's pretty close to purely resistive aside from the cord. 1mH is a good approximation for that.I'll put in a TVS diode and some filter components.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: June 07, 2014, 01:29:00 pm
content: Phaedrus,My concern is to protect the switching MOSFETs from over-voltage.Did you run the simulations?Regards,Jay_Diddy_B

author: Phaedrus
author_rank: Frequent Contributor
timestamp: June 07, 2014, 03:36:25 pm
content: Oh, I thought you were concerned about the load. The mosfets, yeah... I might have to go 600V.