PSWGT-300 Power jack 300W Grid Tie Inverter - Microinverter - Repair

PSWGT-300 Grid Tie Inverter

Power Jack (henceforth referred to as PJ), a Chinese a brand that has risen to notoriety in the past few years, has been churning out cheap (both price and quality) grid tie inverters.

A micro-inverter for the masses
The amazingly cheap prices for Power Jack GTIs meant that almost every man and his dog can afford to invest in a small (micro?) scale grid-feed solar PV system. In fact, products such as this are commonly used in the guerrilla solar movement.

Simply add some solar panels and cables and that's all you'll need to start your green initiatives. There's no need to spend thousands on a much larger, professionally installed system, nor the need to worry about paperwork and having to deal with the utility company in order to get the system connected and switched on. Instead, you start small, with minimal outlay. Should you decide to expand, you can do so gradually.

These PJ inverters are now widely available, especially on ebay. At the time of writing (2011), a typical 300 watt unit costs anywhere from $120 to $200.

A small system feeding up to 130 watts back into the grid via two 80 watt panels with a PSWGT-300 could cost as little as $600.

Efficiency & RFI
Out of the box, these inverters don't quite live up to the manufacturer's efficiency claims of 90% - 95%. The explaination for this is simple: although there is a 4,700uF 35v capacitor across the DC rails (which is fine for the high frequency chopper), the power draw on the secondary (AC) side runs at low frequency (50 or 60Hz). This low frequency severely 'rattles' the 4,700uF cap, leading to efficiencies of around 80%. Worse still, when connected to solar PV panels using more than a few meters of cable, efficiency suffers even more. Solar panels are basically a high impedance voltage / current source, and the impedance of the cables don't help.

There is a simple solution to this: fit a 10,000uF (or higher) capacitor across the solar PV rails, as close as possible to the DC input terminals of the inverter. Ideally, the 10,000uF capacitor should be installed into a spare space inside the GTI's enclosure, using heavy flyleads to connect it to the DC rails. The capacitor provides the much needed low-imedance DC source to let the inverter run closer to its rated efficiency. Even better, use a combination of both an internally fitted capacitor, as well as one across the PV rails outside the inverter. Some have suggested using capacitors as large as 250,000uF for wind turbines, which may seem extreme, but for anything over a few hundred watts, it makes a big difference.

Radiofrequency Interference (RFI) can also be a problem with these units. They were designed to be cheap rather than to pass stringent EMC regulations with flying colours. The AC side is generally OK, but the DC side on a stock standard PJ GTI usually has RFI pouring out, made worse by the cable run to the solar panels (this acts as an antenna; the longer the wire, the worse it is). Some have reported that it 'wipes out AM radio' within a 100 meter radius.

The RFI fix is also a simple affair - install bypass capacitors between the DC rails and the ground (case). Grab two ceramic, MKT or greencap capacitors with a value of at least 0.1uF 100v. Use one capacitor per positive and negative terminal, with one side connected to the terminal, and the other end tagged to the case. Once installed, RFI is virtually cured.

Having said that, the PJ GTI does have a 4.7nF capacitor between the earth/case and negative DC rail, but this is far too small to provide adequete RFI supression. And the supression capacitor is located more towards the AC side, leaving the DC side inadequetely supressed.

Known Problems
These GTIs are well known for 'going bang', blowing their mains fuse due to the MOSFETs shorting out on the high voltage side. The reason for this is due to voltage spikes causing the tracking to go out of sync, causing the MOSFETs to push the wrong way against the mains power source and subsequently blowing them. Switching the AC power to (or plugging/unplugging) the GTI whilst the DC source is present is similar to voltage spikes and there is every chance the MOSFETs will blow.

Always disconnect the DC side before turning on/off the AC side. If there is a grid blackout, the anti-islanding function will work as per usual, but the risk of blowing the MOSFETs will be low due to other loads (appliances) on the circuit pulling the AC line voltage to zero.

Some 300W units do not come fitted with a cooling fan. This severely limits their power handling capacity before overheating. In this case, they should be de-rated to not have more than 100 watts of solar PV input.

Regardless of the rated capacity, it is recommended to not exceed 150 watts of input. Higher input wattages only lead to slightly increased AC output, with most of the energy wasted as heat. Efficiendy drops off steeply above 200W. With 120 watts flowing through, the cooling fan can run continuously. This is a sure sign the inverter is wasting energy.

Important note: Due to the below-average build quality and cost-cutting measures (which sacrifice reliability and safety), PJ products are NOT recommended for the average DIYer unless you are experienced in electronics and repair (particularly in the field of switchmode power supplies). Even their website is full of 'Chinglish', with many Ebay sellers blissfully copying the atrocious product literature for use in their item description. Lines such as "How to Install?" and "How to take away?" are comical and confusing when a simpler "How to Plug in" and "How to unplug" would suffice.

Nonetheless, experienced electronics personnel will still come across some hurdles. Power Jack took extreme measures to protect their designs, which includes sanding/scrubbing the tops of all ICs to remove identifying marks, rendering repair difficult. Sneaky tactics such as this are designed to make the (already unreliable) inverter a 'throw-away' item - i.e. if it breaks, you'll need to buy a new one.... Buyer beware!

After a failure of the MOSFETs, repair is possible. Replacing the offending MOSFET usually restores the inverter's operation, although anecdotally, they have a tendancy to blow again after some time. Some say the inverter is more reliable on 110v than on 240v grids. It could be due to the voltage rating of the MOSFETs. 110v versions may use 500v rated MOSFETS, and the 220v version uses 600v MOSFETS. Where possible, use 900V MOSFETs if available.

When replacing MOSFETs (especially on the AC side) it is prudent to replace them as a group of 4, for when one blows, it can weaken the remaining good FETs, rendering them more likely to blow.

Repair may be worthwhile if you have the necessary experienced and/or knowledge. For what they're worth, you couldn't even buy all the components for any less than the selling price of a PJ inverter. PJ inverters can be of value provided the user knows how to maintain and repair them.


lambcutlet, Thu, 03 Nov 2011 12:28 pm: Reply
since i followed and used some of the info from the topics regarding the powerjack 300w grid tie inverter

Scrubbed component U2 appears to be a PWM controller
part number depends on component manufacturer but heres a short list

U2, onsemi - SG3525A or Fairchild - KA3525A or TI - UC3525A

they are all the same

Q4, Q5, marking 718 is FMMT718 pnp transistor
D5, D6, marking ZS1 is ZHCS1000 schottky diode

reason i worked this out. I wanted to check if the MOSFETs were specced correctly for there application
my gti blew the MOSFETs Q6 & Q1 STP75nf75
terry, Fri, 22 Mar 2013 09:59 pm:
have you experienced any problems with output frequency running at 60hz instead of 50hz i have 1of a pair of units which has been doing this from day one have checked all components and have been unable to solve unless it is being caused byu2 or u4

currywurst, Tue, 22 May 2012 06:04 pm: Reply
Is there any reason to not have a hi-voltage cap to flatten the 300V (or whatever)?
These caps could be taken from PC supplies easily and may improve efficiency since the transformer could be loaded more continuous...

gab, Fri, 27 Jul 2012 09:23 pm: Reply
U3 : 1013D

Luke Tanti, Fri, 09 Nov 2012 06:23 am: Reply
I have a 350W GTI bought from ebay. It stopped working. Inside I found one AC mosfet shorted and a blown AC fuse. I replaced them but still the inverter would only show the red LED on. What can be the problem now??

rlk, Fri, 02 Aug 2013 06:27 pm: Reply
I have a sun 250 g with a failure .
I dont now what kind of component is U4

marwan, Thu, 16 Jan 2014 12:59 am: Reply
i have a 2 gti-500 tie on grid inverter, both work for more than 4 months, later one blown the AC mosfet and the fuse on AC side, i changed the Mosfet it worked for one day and got blown again, later the second inverter got blown with the same fault ,it seems there is something wrong in the design of this inverter it keeps blowing the AC Mosfet and its fuse(5 amp).
my question is it possible to change the Mosfet to a higher voltage and ampere such as changing the original Mosfet MDF18N50 chagne it with IRFP460 will it work, any advice as i am not intended to buy another new one because i feel it is bad design, need advice
John, Wed, 18 May 2016 12:18 pm:
I have a GTI 500 it has four Mosfet F30NM60ND on the AC side maybe F30NM60ND is higher then MDF18N50
John, Wed, 18 May 2016 12:23 pm:
MDF18N50 N-Channel MOSFET 500V, 18 A,

F30NM60ND 30NM60ND 600V 25A
John, Wed, 18 May 2016 12:51 pm:
I got a GTI 300 watt with Mosfet MDF18N50 i think
you should use 30NM60ND 600V 25A in your GTI 500w
MDF18N50 500V 18 A are to low
John, Wed, 18 May 2016 01:09 pm:
I think The Mosfet IRFP460 20A 500V is used in GTI 1000w 230 volt inverters they have bigger kapsels
John, Wed, 18 May 2016 01:20 pm:
The Mosfet FTW40N20A 200v 40A are used in GTI 1000w 110 volt inverter

Peter, Thu, 08 May 2014 12:20 pm: Reply
I'm trying to buy as many of these faulty units as possible to try get some going again for both my self and the public that has been taking for a ride. If your keen to sell faulty units I'm keen to buy I live in Brisbane Australia. Don't know if I can leave my email address here but I'll try. or my phone number is 0459 667 483 phone is only turned on 9-5 weekdays.

Peter, Sat, 31 May 2014 07:27 am: Reply
I just got a PSWGT=300 of ebay as faulty and thought I would have a go at fixing it. I noticed when it arrived that the heat sink clamp was missing on the mosfets. There was also no insulating washers on the mosfets or the doides so I removed the board and done some research on the net and the thing I have noticed is thatI only seem to have four diodes fitted to my unit and One choke there is also one link on the 110v connection. all the images I can find on the net seem to show both chokes fitted and the eight diodes can any body advise on this. oh it looks from the board that these parts have never been installed?

Peter, Sat, 31 May 2014 08:16 am: Reply
Peter, sms or email me a photo of your board, I think we have the same unit, we can use each others to cross check measurements.
Peter, Sat, 31 May 2014 10:06 pm:
Hi how do I send an email do I need your address? sorry I am new to these sites. I should say that on mine when I removed the board so the devices where not shorting I connected a 12 battery to the unit and plugged it in and after a while the three green leds started to chase and observing a power consumption meter I have on my meter supply it did drop by about 30 watts. I then put a couple of the 12 battery's in series and the leds chased faster and then after about 2 min it went to the red led. I put the 12v battery back and still functions ok. I will take a picture latter as I am off out and need to take it apart again if you can advise how to email it to you.
Thanks for your response
Peter, Tue, 03 Jun 2014 03:25 pm:
0459 667 483

Steven, Mon, 09 Jun 2014 11:27 pm: Reply
I had a FET blowing, when I switched of a fluorecent tube (with classic inductive ballast)
It generates a back emf pulse, that instanty blew a FET. The FET is replaced, and I added 0,1µF "X2" cap paralell on the fluorecent armature, to suppress the spark. There was no cap (older armatures) A newer armature did have a cap fitted.

Hope it helps me and others!

gman, Fri, 22 Aug 2014 12:22 pm: Reply
I read 'Efficiency & RFI' about adding 10,000uF (or higher) capacitor across the solar PV rails.
My question is would a 22000uF 25V capacitor work with my 18 Volt solar panels ?

Fernando, Mon, 15 Sep 2014 04:39 pm: Reply
So by going with FH I cut down my fault domain from 210w to 15w and my setsym can survive panels losses without impacting the overall setsym capacity much. Also, the grid tie setsym dies when the power is out and thus I also wanted battery backup, so now I have 1Kw of HF panels and 2Kw of DuPont panels going to batteries. Most people won't think about what will happen if to their setsym capacity if they lose one panel I do.I built for survivability and not for cost savings

John, Wed, 17 Sep 2014 08:09 am: Reply
Agreed, the big question is how much power did he use from the time he shot the meter to the time he got power<a href=""> tnreud</a> off.Thus in order to get it to run backwards 1 kw he would have had to leave it on much longer than the 40 min he said in the video my guess is the meter would have ran backwards as it already indicated the direction of current was going in the other direction (Back to the Grid)

Paul, Thu, 18 Sep 2014 06:04 pm: Reply
Wow, Dan THANK YOU so much for that comprehensive <a href="">pruodct</a> review and grid tie inverter lesson. What you did in that 12 min vid (which took perhaps a whole day to produce) was so informative that I immediately subscribe to your channel. For the past few days I've been asking all sorts of questions about the GTIs and getting its and bits of info along the way, but your single video provided all of the info plus more. Thanks

luisascencio, Wed, 11 Mar 2015 06:50 am: Reply
puedes darme el valor de Q15
Greetings, you may give me the numbering of Q15, from and through

Marcos, Mon, 01 Jun 2015 12:00 am: Reply
They missed a chip when they where sanding down the ic's on my board

U2 is a texas instrument chip

A36Y G4

Hope this helps someone.

Geert, Mon, 18 Apr 2016 04:27 am: Reply
Hi, my PSWGT-300 does not power on anymore, not fet's or diodes are damaged.
At the moment there is 8 volt on the input the RED Led wil go on, this with or without plugged into the grid.
Any idea what this problem could be?

Terry in Ohio, Fri, 10 Jun 2016 05:25 am: Reply
I have a PSWGT600 that I'm restoring for a friend and need parts info. Anyone have this unit?


Terry in Ohio

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