Heating up of Charger IC

[FreshmanNewbie]

I'm using this charger IC - MCP73842T. The IC is driving my PMOS gate about 2V with input voltage Vdd = 11.5V. The datasheet suggests a higher voltage (4.5V to 7V). With my charging current of 0.5A, the MOSFET is getting very hot.

What's the main reason my MOSFET is getting hot? Is the DRV pin of the Charger IC not driving my MOSFET properly?

Since the MOSFET part mentioned in the evaluation guide is obsolete, I need to select another MOSFET and see if it helps.

I have selected this alternative MOSFET. Any suggestions whether it would help?

If not, please let me know the parameters which I need to check for selecting a new MOSFET to ensure it does not heat up.

 

[KlausST]

Hi,

Headline says:
* Heating up of CHARGER IC

your text says:
* MOSFET is getting hot

What now?

And in case you mean the MOSFET heating up:
You can go back to some of my replies to your posts where I already told you to accept the basic laws of physics:
ANY linear circuit has a power dissipation: P = I * V.
No matter what electronics device it is: Diode, MOSFET, resistor, BJT, zener, LED ..... you can´t avoid this on linear circuits.

--> That´s why one uses the "switching circuits" that don´t suffer from this high power dissipation.

Klaus

 

[danadakk]

Can you post a schematic and screen shots or table of gate drive, Vds, and Idrain (you need
a shunt R in source or drain path to sample to compute current).

As a review look at datasheet, make sure your max's all meet absolute max ratings,
with some margin (say 20% or more) and SOA curves also met.

Your gate drive of only 2V not turning fet on hard, so its dissipating a lot of power.
The application is putting MOSFET in linear mode.

Pd = Idrain² x Rdson

It would also be helpful to post well focused pics of layout/prototype, heat sink

 

[FreshmanNewbie]

Hi,

Headline says:
* Heating up of CHARGER IC

your text says:
* MOSFET is getting hot

What now?

And in case you mean the MOSFET heating up:
You can go back to some of my replies to your posts where I already told you to accept the basic laws of physics:
ANY linear circuit has a power dissipation: P = I * V.
No matter what electronics device it is: Diode, MOSFET, resistor, BJT, zener, LED ..... you can´t avoid this on linear circuits.

--> That´s why one uses the "switching circuits" that don´t suffer from this high power dissipation.

Klaus

My apologies.

It is heating up of MOSFET.

Can you tell me what parameters to look for in the alternate MOSFET that I need to select? Whether the new MOSFET will help?

 

[KlausST]

Whether the new MOSFET will help?

Either you did not read or not comprehend my post ...

Klaus

 

[danadakk]

Forum needs to see your design, eg. pics of its PCB, layout, schematic, physical size of heat sink.....

Forum tries to assemble all design info before making recommendations. Otherwise its
equivalent to using my Car mechanic to do open heart surgery on me.

 

[FreshmanNewbie]

Either you did not read or not comprehend my post ...

Klaus

I did read. I understand that, yes. The heating issue cannot be avoided. But how can it be reduced to some extent. I would like to know what design changes would lead to reduced heat. Hence, I thought another MOSFET (with better specs? but not sure what specs I need to look) would help? Am I going in the right direction?

 

[danadakk]

You are operating MOSFET as a variable R in the charging circuit. So there is no
"better" MOSFET for circuit.

But your heat can be handled with a different package with a lower thermal R,
or a bigger heatsink. And/or more airflow on heatsink will help lower overall
total Thermal resistance from die to ambient. Hence lowering did junction T
allowing MOSFET to run a little cooler.

Regards, Dana.

 

[KlausST]

Am I going in the right direction?

No. Definitely not.
How much more clear can I be?
It does not matter wich MOSFET you choose, power dissipation will always be the same!

You can use a small MOSFET or big MOSFET --> total power dissipation will be the same.
You can use a BJT, normal or darlington --> total power dissipation will be the same.
You can use an additional resistor --> total power dissipation will be the same.
You can use an additional diode --> total power dissipation will be the same.
You can use a SICMOS, or an IGBT --> total power dissipation will be the same.

Klaus

 

[FreshmanNewbie]

No. Definitely not.
How much more clear can I be?
It does not matter wich MOSFET you choose, power dissipation will always be the same!

You can use a small MOSFET or big MOSFET --> total power dissipation will be the same.
You can use a BJT, normal or darlington --> total power dissipation will be the same.
You can use an additional resistor --> total power dissipation will be the same.
You can use an additional diode --> total power dissipation will be the same.
You can use a SICMOS, or an IGBT --> total power dissipation will be the same.

Klaus

So, you mean to say, the difference between input and output, (11.5V - 8.4V)*0.5A = 1.55W, will be dissipated in my MOSFET?

And regardless of whichever MOSFET I take, this is going to be the same issue?

Then, the only other options are either to use a different charger IC, or to install a heatsink on the charger IC.

Since, selection of any other MOSFET will provide the same heat dissipation no matter how good the Rdson or Vgs specs are , right?

 

[KlausST]

I guess there is no need to repeat again.

Klaus

 

[D.A.(Tony)Stewart]

I'm using this charger IC - MCP73842T. The IC is driving my PMOS gate about 2V with input voltage Vdd = 11.5V. The datasheet suggests a higher voltage (4.5V to 7V). With my charging current of 0.5A, the MOSFET is getting very hot.

What's the main reason my MOSFET is getting hot? Is the DRV pin of the Charger IC not driving my MOSFET properly?

Since the MOSFET part mentioned in the evaluation guide is obsolete, I need to select another MOSFET and see if it helps.

I have selected this alternative MOSFET. Any suggestions whether it would help?

If not, please let me know the parameters which I need to check for selecting a new MOSFET to ensure it does not heat up.

Vgs(th) = -1 to -2.5 V = Vt

A low Rds depend on Vgs being at least twice the actual value, pref. >250% x Vt.
What does gate= 2V mean ? Vgs = 11.5 -2 ?

Define your heatsink.

 

[Calm down]

Although the MOS transistor has a narrow amplification working area, if the driving is insufficient, it may still operate in the amplification area, which means that there is continuous current flowing and the impedance is higher when current needs to flow, causing it to generate abnormal heat.

 

[D.A.(Tony)Stewart]

FET heat rise is Rth * Pd.

Pavg = (per cycle) Ps, static power + Pd, dynamic power from capacitive load effects

Ps = Vds (t) * Ids (t) (integrated over cycle)

Ids = beta/2 * (Vgs - Vt)^2. (Vt=Vgs(th) min to max, beta related to RdsOn)

 

[KlausST]

Pavg = (per cycle)

Would you please define "cycle" .. on a linear circuit like the OP ones.

dynamic power from capacitive load effects

What "dynamic power" ... do yo consider on a DC working circuit?

integrated over cycle

... again.


Klaus

 

[D.A.(Tony)Stewart]

Would you please define "cycle" .. on a linear circuit like the OP ones.


What "dynamic power" ... do yo consider on a DC working circuit?


... again.


Klaus

AC Power loss is always averaged over any integer of N cycles with static losses due to RdsOn and dynamic losses due to switching reactive loads if PWM or PFM is used.

Change IC to https://ww1.microchip.com/downloads...ts/MCP1631-HV-V-VHV-Data-Sheet-DS20002063.pdf

But i was thinking of a generic application as you might know, as this is quasi-DC. with far too much voltage drop across the series pass FT conductor.

Perhaps I was thinking of a better solution if the input voltage is still fixed and that is to use PWM and average the sensed current as a buck current regulator with RLC filtering.

--- Updated Mar 6, 2025 ---

I'm using this charger IC - MCP73842T. The IC is driving my PMOS gate about 2V with input voltage Vdd = 11.5V. The datasheet suggests a higher voltage (4.5V to 7V). With my charging current of 0.5A, the MOSFET is getting very hot.

What's the main reason my MOSFET is getting hot? Is the DRV pin of the Charger IC not driving my MOSFET properly?

Since the MOSFET part mentioned in the evaluation guide is obsolete, I need to select another MOSFET and see if it helps.

I have selected this alternative MOSFET. Any suggestions whether it would help?

If not, please let me know the parameters which I need to check for selecting a new MOSFET to ensure it does not heat up.

Series V*I= power loss makes your choice of IC unusable for the FET. The PWM buck version has either the HV or UHV suffix for switch mode from a higher voltage .link above.

To choose your FET, Vgs must be at least 250% times the off threshold (my label) which is called Vt or Vgs(th) when it is a std. type of Vt= 2 ot 4V so that is 10V worst case.

Low threshold types or "logic level FETs only need 200% times Vt.

These are Rules of thumb using worst case Vt max or Vgs(th) max