In this post we try to understand how the internal body diodes of MOSFETs could be exploited for enabling the charging of battery through the same transformer which is being used as the inverter transformer.
In this article we will investigate a full bridge inverter concept and learn how the in-built diodes of its 4 MOSFETs could be applied for charging an attached battery.
What is a Full Bridge or H-Bridge Inverter
In few of my earlier posts we have discussed full bridge inverter circuits and regarding their working principle.
As shown in the above image, basically, in a full-bridge inverter we have a set of 4 MOSFETs connected to the output load. The diagonally connected MOSFET pairs are alternately switched through an external oscillator, causing the input DC from the battery to transform into an alternating current or AC for the load.
The load is normally in the form of a transformer, whose low voltage primary is connected with the MOSFET bridge for the intended DC to AC inversion.
Typically, the 4 N-channel MOSFET based H-bridge topology is applied in full bridge inverters, since this topology provides the most efficient working in terms of compactness to power output ratio.
Although using 4 N channel inverters depend on specialized driver ICs with bootstrapping, yet the efficiency overweighs the complexity, hence these types are popularly employed in all modern full bridge inverters.
Purpose of MOSFET Internal Body Diodes
The internal body diodes present in almost all modern day MOSFETs are primarily introduced to safeguard the device from reverse EMF spikes generated from a connected inductive load, such as a transformer, motor, solenoid etc.
When an inductive load is switched ON through the MOSFET drain, electrical energy gets stored instantaneously inside the load, and during the next moment as the MOSFET turns OFF, this stored EMF is kicked back in the reverse polarity from MOSFET source to drain, causing a permanent damage to the MOSFET.
The presence of an internal body diode across the drain/source of the device thwarts the danger by allowing this back emf spike a direct path through the diode, thus safeguarding the MOSFET from a possible breakdown.
Using MOSFET Body Diodes for Charging Inverter Battery
We know that an inverter is incomplete without a battery, and an inverter battery inevitably requires charging frequently to keep the inverter output topped-up and in the standby condition.
However, charging a battery requires a transformer, which needs to be a high wattage type to ensure optimal current for the battery.
Using a additional transformer in conjunction with the inverter transformer can be quite bulky and costly too. Therefore finding a technique in which the same inverter transformer is applied for charging the battery sounds extremely beneficial.
The presence of the internal body diodes in MOSFETs fortunately makes it possible for the transformer to be switched in the inverter mode and also in the battery charger mode, through some easy relay changeovers sequences.
Basic Working Concept
In the diagram below we can see that, each MOSFET is accompanied with an internal body diode, connected across their drain/source pins.
The anode of the diode is connected with the source pin, while the cathode pin is associated with the source pin of the device. We can also see that since the MOSFETs are configured in a bridged network, the diodes also become configured in a basic full-bridge rectifier network format.
A couple of relays are employed which implement a few quick changeovers for enabling the grid AC to charge the battery via the MOSFET body diodes.
This bridge rectifier network formation of the MOSFET internal diodes actually makes the process of using a single transformer as an inverter transformer and charger transformer very straightforward.
Current Flow Direction through MOSFET Body Diodes
The following image shows the direction of current flow through the body diodes for rectifying the transformer AC to a DC charging voltage
With an AC supply, the transformer wires change their polarity alternately. As shown in the left image, assuming the START as the positive wire, the orange arrows indicate the flow pattern of current via D1, battery, D3 and back to the FINISH or the negative wire of the transformer.
For the next AC cycle, the polarity reverses, and the current moves as indicated by the blue arrows via body diode D4, battery, D2, and back to the FINISH or the negative end of the transformer winding. This keeps repeating alternately, transforming both the AC cycles to DC and charging the battery.
However, since MOSFETs are also involved in the system, extreme care has to be exercised to ensure that these device do not get damaged in the process, and this calls for a perfect inverter/charger changeover operations.
Practical Design
The following diagram shows a practical design set up for implementing MOSFET body diodes as a rectifier for charging an inverter battery, with relay changeover switches.
To ensure 100% safety for the MOSFETs in the charging mode and while using the body diodes with the transformer AC, the MOSFET gates must be held at the ground potential, and completely cut-off from the supply DC.
For this we implement two things, connect 1 k resistors across the gate/source pins of all the MOSFETs, and a put a cut-off relay in series with the Vcc supply line of the driver IC.
The cut-off relay is an SPDT relay contact with its N/C contacts connected in series with the driver IC supply input. In the absence of AC mains, the N/C contacts remain active allowing the battery supply to reach the driver IC for powering the MOSFETs.
When mains AC is available, this relay changes over to the N/O contacts cutting off the IC Vcc from the power source, thus ensuring a total cut off for the MOSFETs from the positive drive.
We can see another set of relay contacts connected with the transformer 220 V mains side. This winding constitutes the output 220V side of the inverter. The winding ends are connected with the poles of a DPDT relay, whose N/O an N/C contacts are configured with the mains grid input AC and the load respectively.
In the absence of mains grid AC, the system works in the inverter mode, and the power output is delivered to the load via the N/C contacts of the DPDT.
In the presence of an AC grid input, the relay activates to N/O contacts allowing the grid AC to power the 220V side of the transformer. This in turn energizes the inverter side of the transformer and the current is allowed to pass through the body diodes of the MOSFETs for charging the attached battery.
Before the DPDT relay is able to activate, the SPDT relay is supposed to cut off the Vcc of the driver IC from the supply. This slight delay in activation between the SPDT relay and the DPDT relay must be ensured in order to guarantee 100% safety for the MOSFETs and for the sound operations of the inverter/charging mode via the body diodes.
Relay Changeover Operations
As suggested above, when mains supply is available the Vcc side SPDT relay contact should activate a few milliseconds before the the DPDT relay, at the transformer side. However, when the mains input fails, both the relays must switch OFF almost simultaneously. These conditions could be implemented using the following circuit.
Here, the operational DC supply for the relay coil is acquired from a standard AC to DC adapter, plugged with the grid mains.
This means, when grid AC is available, the AC/DC adapter powers ON the relays. The SPDT relay being connected directly to the DC supply activates quickly before the DPDT relay can. The DPDT relay activates a few milliseconds later due to the presence of the 10 ohm and the 470 uF capacitor. This ensures that the MOSFET driver IC is disabled before the transformer is able to respond to the grid AC input at its 220 V side.
When mains AC fails, both the relay switch OFF almost simultaneously, since the 470uF capacitor now has no effect on the DPDT due to the series reverse biased diode.
This concludes our explanation regarding using MOSFET body diodes for charging an inverter battery through a single common transformer. Hopefully, the idea will allow the many hobbyists to build cheap, compact automatic inverters with built-in battery chargers, using a single common transformer.
Also sir,can any mosfet charge? Also how should one match battery charging amperes and the mosfet amperes. Can high mosfet switching amperes have effect to the battery and or the mosfets during charging?.I have tried the charging side and it’s working fine but for my sake I used irfp1404 . The circuit did well for sometime like 2hrs but blew mosfets on one side. My transformer has an output of 0-20v 50A charging 2×12v 200Ah batteries. I connected the gates to source using a 10k resistor. What do you think is wrong or what might have I done wrong?
I am glad it is working…The back diodes are provided for supporting low current spikes, it is not provided for consistent high current use, therefore for the above explained battery charging application, we must reinforce the body diodes with external high current diodes.
MOSFETs have no role in this application and will remain unresponsive and safe, unless the diode functioning breaks or stops working
Sir so when the back diode breaks causes the whole mosfet to be affected? Again sir advise me,say I need 50A min charging current,how many amperes of a diode should I use for each mosfet for the reinforcement?. Or what if I parallel more mosfets to deal with the issue of back diode amps?
Evans, the diodes must be rated at two times the estimated charging current value.
Okay sir
Sir I have read a certain article on Google,the article says that the mosfet current rating is the same as the mosfet’s body diode,an yet to understand with regard to what you had told me earlier that mosfets body diodes are provided for supporting low current spikes, please where can I send you the article so you can go through so you can advise? Your address please and I will appreciate.
Evans, It was just an assumption because I thought it did not make sense to put a body diode with the same current specs as the MOSFET. But it seems the diodes are being introduced not just for spike protection but also for a full-fledged use as reverse diodes. the following datasheet shows that the mosfet Id and the diode current specs are actually similar
https://www.vishay.com/docs/91560/sihw47n60ef.pdf
That’s right sir,now what could lead to blowing of one side of mosfets infact 4 of irf1404 when charging 1 n70 12v battery? Just like that. But in inveter mode,the inverter function very well in no heat is felt. Even when charging,no heating at all but one side of mosfets just blows off,this has happened to me twice?? Any help or anything am not doing right?? Coz I did as you directed interms of mosfets protection.
I think you should try disconnecting the MOSFETs gates completely from the drive source, and have a 1k resistor connected across gate/source of each MOSFETs. With this configuration try applying their reverse diodes for the rectification of the transformer AC and see how it works.
Fine Sir,let me try that and will be back with feedback soon
OK!
I like it sir, very clear and up to task. Now I see both the coils of the two relays connected to one supply,from the diagram above on the change over operation,it’s clear that both the DPDT relays will get energized from the delay supply. My worry is that there will be a delay in connecting grid to the load. How do we deal with that? Or we remove the delay?
No problem Evans, you can remove the delay effect, it is introduced for ensuring guaranteed safety to the MOSFETs, but removing it might still allow the circuit to work without issues.
Sir,think about this ,what if I introduce another coil to the transformer for the charging purpose alone and another relay to work with the delay alone,such that in the presence of mains supply,grid is connected with the loads first with the oscillatilor supply being cut off at the same time the coil from the transformer that outputs the inverter power is disconnected from the load,then a small delay before the charging coil is engaged to the mains. How about that.
Evans, that seems to be a good idea, which will also help to get rid of the boost converter circuit.
Thanks sir
Good job sir, atleast you promised and delivered. Now my other questions are:
1. From the diagram above,I can see in the presence of mains,the grid is connected to the transformer for charging after the delay,now what about for the appliances,coz I would like in the presence of mains,also the appliances get connected to grid uninterrupted?.
2. What will be the effect of instead of using 1k between gate and source I use a 10k resistor?.
3. Full bridge Inverters involving pwm have their transformer voltage almost half of the battery,now how will the battery be charged to full charge in such a situation,any incorporated diagram will be appreciated.
4. Also sir,what about current limiting , float charging and or auto fully charged cut off?.
Sir I have already constructed the inverter and it’s working,just waiting for your response for further upgrade.
Thank you Evans, For the appliances you will have to use another DPDT relay which will simultaneously connect the load with the grid input. The poles of the relay will connect with the load, and the N/O contacts with the grid input.
10k will also do, but lower value will provide better contact with the source line, although the value may not be critical due to high impedance of the gates.
For transformers rated 50% of the battery might require a boost converter, and another set of DPDT contacts for diverting the DC from the body diodes through the boost converter.
The boost converter output can have all the necessary features such as auto cut off float charging etc in case these are required.
Is it possible you design one booster converter with all the features say float charging, current limiting extra and show how to connect to the circuit above? I’ll really appreciate
Again thanks for your endless help,I even lack words to describe you but all I can say is God almighty bless you and Grant you your desires.
Thanks Evans, the cut off and the float can be introduced at the output of the boost charger, but I cannot design the boost charger, you will have to refer to some reliable source for the circuit such as ti.com etc for the drawing.
Hi sir swagatam, there’s an observation am seeing. I see a situation whereby if I introduce another DPDT relay that will connect the load with the grid simultaneously,that it will renter the first DPDT relay useless since we have a delay meaning the transformer will get mains from the grid through the relay tht connects the load and grid because of the delay. So sir look at that issue and advise. Inshort sir advise on how to Energizer the second DPDT relay coil,is it by the direct vcc or through the delay? Av an issue configuring this. Thanks in advance.
Hi Evans, the delay will be in milliseconds only, still I will check the working by drawing the connections, and see if there’s a possible conflict.
Again sir,from the above circuit with delay, how can I increase the delay time to about 5seconds?
for 5 seconds you may have to employ a transistor driver stage, with an appropriately calculated base RC network
Fine Sir,will be waiting to hear from you soon
Hi Evans, here’s the drawing for the DPT contact which will make sure that the load gets connected with the grid AC during inverter charging mode, and when the grid AC is available:
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