NPN power transistor with built-in diode? - Page 1
Forum section: electronics / projects

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: February 16, 2014, 02:06:37 am
content: Hi,First post here, hope it's in the right category.I arrived home this afternoon to notice that my Koolatron 8-bottle wine cooler was off. After the usual basic troubleshooting, I took it apart to measure some things.There is one board in there, a Chinese SMPS with a temperature regulator.It's a Hanny product, model # HYS5A60-JH, manufactured in 2011. Their website doesn't list that #, but several similar ones. (I emailed for a schematic, we'll see...)http://www.hanny.com.cn/en/pd-2xg.aspThe fuse is blown, and most of the input-side semiconductors are shorted. The input diodes and transistors are 0 ohm shorts.I think the SMPS is a full-bridge converter from what I've traced out so far, and from the fact I can see the transformer coupling to the switching transistors.I was expecting MOSFETS, but turns out they are NPN with built-in diode across C-E. This is new to me, I've never heard of bipolar transistors with a built-in (non-isolated) diode before.It's a "Continental Device India Limited" (never heard of them!) 13005D, which I thought was a NXP PHE13005.I can't find anything online similar to this part. I have some 2SC3039 transistors, but the big question is what is the job of these C-E diodes and can I just whack something across the C-E to do that job? I'm not too familiar with SMPS topologies.Or if you have a equivalent part number.Thanks

author: vk6zgo
author_rank: Super Contributor
timestamp: February 16, 2014, 03:42:24 am
content: They are not uncommon in horizontal output stages of CRT type TV sets & Monitors,& in the SMPS of such devices.It's too long since I've worked on them so I'll have to look up a reference.

author: N2IXK
author_rank: Frequent Contributor
timestamp: February 16, 2014, 03:57:39 am
content: High voltage NPN transistors with a C-E diode were fairly common in the horizontal output stages of TV sets in the last 20 years or so of the CRT era. They often included a 50-100 ohm resistor from B to E, as well. Typical type would be 2SD2586, but there are MANY others, both TO-3 metal and various plastic packages.The diode is referred to as a "damper", to protect the transistor from excessive voltage spikes when switching inductive loads, like transformer windings...

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: February 16, 2014, 05:58:08 am
content: Guess I should have googled a bit more, by simply putting the D as a prefix you get PHD13005 instead of PHE13005, in stock at Digikey. Strangely, only one source on eBay, and far too expensive.

author: dannyf
author_rank: Super Contributor
timestamp: February 16, 2014, 11:52:12 am
content: It is a Motorola / onsemi mje13005 (and its gazillion other variations). Extremely inexpensive and largely available everywhere.Its brother, 13001 or 13002, practically lines in every CFL lightbulb.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: February 16, 2014, 06:07:03 pm
content: With the built-in diode, that's what was harder to find. You gotta put the "D" at the right place in the part number!

author: Circlotron
author_rank: Super Contributor
timestamp: February 17, 2014, 04:39:07 am
content: Quote from: Alex Eisenhut on February 16, 2014, 02:06:37 amWith the two transistors stacked one above the other, the diodes stop the halfway point that drives the transformer going any higher than the top collector or lower than the bottom emitter. If the diodes were not present, if for example the voltage swung too low it would forward bias the lower C-B junction and consequently reverse bias the the lower B-E junction. The B-E junction will only tolerate about 5 volts or so before it begins to avalanche like a zener, and push it too hard and ->Short answer, it protects the transistors.

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 17, 2014, 05:03:40 am
content: If it's 12V, just replace the whole thing with an ATX PSU.

author: Richard Head
author_rank: Frequent Contributor
timestamp: February 17, 2014, 05:52:51 am
content: With the two transistors stacked one above the other, the diodes stop the halfway point that drives the transformer going any higher than the top collector or lower than the bottom emitter. If the diodes were not present, if for example the voltage swung too low it would forward bias the lower C-B junction and consequently reverse bias the the lower B-E junction. The B-E junction will only tolerate about 5 volts or so before it begins to avalanche like a zener, and push it too hard and -> Short answer, it protects the transistors.Quite correct. The diodes are fast recovery and clamp any voltage over/undershoot to the rails. All MOSFETs have them inherent in their structure so no additional diodes are required.The reason bipolars are used as apposed to MOSFETs is that they are a fraction of the price of a MOSFET since the amount of silicon used is much less.Switchmode converters using bipolar transitors need to be carefully designed to ensure that the device doesn't experiance second breakdown under certain circuit conditions. Sometimes even when the device is used within its safe operating area some devices seem to fail without warning.The mje13005D is very common so shouldn't be a problem obtaining.Dick

author: vk6zgo
author_rank: Super Contributor
timestamp: February 17, 2014, 08:07:32 am
content: Quote from: Richard Head on February 17, 2014, 05:52:51 amIt probably isn't designed just for that application.With similar transistors in a horizontal output stage,the internal diode is used as a "damper diode".Same sort of thing,but only one transistor involved.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: February 17, 2014, 04:35:19 pm
content: I read Microchip's AN1207 to familiarize myself with SMPSes. So I was wrong, it's a half-bridge design. I think the root cause of the failure was the input capacitors, after removing them and "testing" them quickly with a simple resistance check, one of them has the familiar RC charging behavior all the way to megohms, the other one stays stuck in the 100s of kohms, I guess it started leaking more at its working voltage, blew some input diodes which shorted the AC straight across the whole input side taking the two transistors with it, and then blowing the fuse. It looks like lightning struck inside the fuse.I also noticed they left one opamp of the LM358 (on the temp sensing circuit) unconnected, I will connect it as a unity gain buffer to 1/2vcc or something like that. Bad design practice to leave stuff floating, it would have cost nothing more to add the connection. I mean it's ugly, they really left the pins floating around the PCB, there isn't even copper! Just holes in the PCB.And no, I can't replace it with a PC power supply, it's a fridge! Where would the temperature regulation come from?And also no, MJE13005D aren't that easy to find. None at DK or Mouser or eBay. Certainly none at the (few) local electronics stores left here. The only reason I had 2SC3039s in my parts is because many years ago I repaired a monitor that used them.Anyways, I ordered new caps, diodes and transistors (PHD13005), and will power up the new parts in the board with my Variac.I fully expect the thing to blow up again in another 2-3 years. See you then?