Driver Circuit for the Postive Buck Boost Circuit

I have been out of the power electronics design for at least a year cause of me spending time in the particular robotics based project which I cannot disclose details of in public.

The said project is almost getting over and now only the power part is left as the controllers designed. Hence I thought I will use this opportunity to let myself come back again into the power electronics designing.

So I start with the drive circuit for any MOSFET. I am in the learning phase but will have to finish and design the part within a week. Hence what I provide may be something wrongly understood or perhaps misinterpreted.So if you come across anything like that please do tell me as  comments and that would help me so much.

I am using the ST electronics Application note for gate circuits design for MOSFET/IGBT.I will provide the link to the document at the end of post.

Update 1
But as I started reading the document started on the highly intelligent description of the mosfet inner working and straight off jumped on to driver design and started describing design of different drive circuits. I was not satisfied it because of as follows.

First off I want to know why the requirement of circuit named gate drive circuit for the Gate driving.

Because before designing something I need to know the purpose of  such a circuit which no paper think was important to include in the application note.

So again I hoped around  the internet sphere and came up with Microchip application note matching gate characteristics with drive circuit.

So here is what the Microchip application note taught me(Very good notes).

 To make a generalized statement about matching a MOSFET driver to a MOSFET based on voltage/current ratings or die sizes is very difficult, if not impossible.

As with any design decision, there are multiple variables involved when selecting the proper MOSFET driver for the MOSFET being used in your design.

Parameters such as the following will take effect.

1. Input-to-output propagation delay,
2. Quiescent current, 
3. Latch-up immunity 
4. Driver current rating must all be taken into account
5. Power dissipation of the driver (will also effect your packaging decision and driver selection)

AD for Microchip (Come on AD is okay they gave this much knowledge free that my college did not)

*Microchip offers many variations of MOSFET drivers in various packages, which allows the designer to select the optimal MOSFET driver for the MOSFET(s) being used in their application.

Since the last ones influences the package and driver selection we will start with the power dissipation calculation first

POWER DISSIPATION IN MOSFET DRIVER(5)

*Driver current rating will not effect the power dissipation of the driver

(Charging and discharging the gate of a MOSFET requires the same amount of energy, regardless of how fast or slow ( rise and fall of gate voltage) it occurs as per their note.

**(But I think it effects the power dissipation but not the energy or heat dissipation).

Therefore, the current drive capability of the MOSFET driver does not effect the power dissipation in the driver due to the capacitive load of the MOSFET gate.)

It also discusses how to match MOSFET driver current drive capability(4) and MOSFET gate 
charge based on desired turn-on and turn-off times of the MOSFET in the later sections(1).

This Application Note discusses the details of MOSFET driver power dissipation in relation to 

1. MOSFET gate charge 
2. Operating frequency

There are three elements of power dissipation in a MOSFET driver:

1.  Power dissipation due to the charging and discharging of the gate capacitance of the MOSFET.

         EQUATION 1: 

    Pc = Cg * Vdd^2*F

   Where: 

   MOSFET Gate Capacitance = Cg 

   Supply Voltage of MOSFET Driver (V) = Vdd 

   Switching Frequency = F

2. Power dissipation due to quiescent current draw of the MOSFET driver(2).

PQ = (IQH ´ D + IQL ´ (1 – D)) ´ VDD

Where:

IQH   =  Quiescent current of the driver with the input in the high state

D      =  Duty cycle of the switching waveform 

IQL    =  Quiescent current of the driver with the input in the low state

3. Power   dissipation   due   to   cross-conduction (shoot-through) current in the MOSFET driver.

PS = CC ´ F ´ VDD

Where:

CC  =  Crossover constant (A*sec)

As deduced from the equations above, only one of the three elements of power dissipation is due to the charging and discharging of the MOSFET gate capacitance. This portion of the power dissipation is typically the highest, ***especially at lower switching frequencies.

In order to calculate a value for Equation1, the gate capacitance of the MOSFET is required. 

Gate capacitance of a MOSFET = 

Gate-to-source capacitance + Gate- to-drain capacitance (Miller Capacitance).

***

Common mistake is to use the Input capacitance rating of the MOSFET (CISS) as the total gate capacitance of the MOSFET.  

***

So the  proper  method  for  determining  gate capacitance is to look at the  "Total Gate Charge (QG) "in the MOSFET data sheet. 

This information is typically could be found from the following two information from the MOSFET datasheet provided by the company.

1. Electrical Characteristics table and
2. Typical characteristics curve 

Electrical Characterstics table would have listed the Total gate charge capacitance at particular Vgs lets say =10V , Vds = 400V using the Q = C * V formulae.

But to customize it to our requirements we have to make use of the Figure 1 in the datasheet which is a typical characterstics curve for the that MOSFET.

In here we have to find the curve corresponding to our required Vgs and Vds first as per our requirements. After finding the curve required next we will have to find the corresponding Total gate charge of the  MOSFET from the Y-axis.

Now we could find the capacitance by simply dividing the total gate charge by the gate to source voltage.

Thus the power dissipation at the mosfet due to the gate capacitance charging and discharging is given by,

Pc = Cg * V^2 * F

Here F is the frequency and V is the gate source voltage and the power dissipation varies with both the voltage alone as evidenced here.

POWER DISSIPATION DUE TO CROSS CONDUCTION( ie; Shoot through)
Now to find the power dissipation due to the  Driver cross-conduction( ie; shoot through current)

First of check the Crossover Energy vs Supply Voltage graph and find the crossover constant(CC) has to be found then inorder to find the  power dissipation due to the cross conduction according to the following formulae ;

Ps = CC * F * V

This power consumption is quite low and insignificant(as per Microchip) as far as the voltage and frequency is low.

But with increase in the current drive capability of the driver this data becomes important and help decide to choose the package in which the chip is to be used for the driving purposes.

DYE SIZE EFFECT ON GATE CAPACITANCE

The dye size is expressed in Hex.

General rule within in a silicon technology is that as dye size increases the gate capacitance increases.


PEAK CURRENT DRIVE REQUIREMENT

The factors found till now will only help in the calculation of the power dissipation due to the internal and external factors.

Matching the MOSFET driver to the MOSFET in the application will primarily be based on how fast the application requires the power MOSFET to be turned on and off (rise and fall time of the gate voltage). The optimum rise/fall time in any application is based on many requirements, such as EMI (conducted and radiated), switching losses, lead/circuit inductance, switching frequency, etc.

The speed at which a MOSFET can be turned on and off is related to how fast the gate capacitance of the MOSFET can be charged and discharged. The relationship between gate capacitance, turn-on/turn-off time and the MOSFET driver current rating can be written as:

dT =( dV * C)/ I

assuming the driver circuit is supplied by constant current source where dT is the turn on/turnoff time.

Now all this done I wanted to find a MOSFET switch that meet the requirement, which is to afford voltage above 45V and current of upto 41A so after little searching I fixed IRFZ48 to design the driver circuit.

So lets begin the design,

Mosfet gate charge    = 85nC
Mosfet gate Voltage  = 12V
Turn on / Turn off time = 40ns

Therefore I = dQ/dT

I = 85nC/40ns =2.125 A

Hence the driver selected must of the above current driving capability or higher.

Update 2

Any external resistance between the MOSFET driver output and the gate of the power MOSFET will also need to be taken into account, as this will reduce the peak charging current supplied to the gate

capacitance. This drive configuration is shown in Figure 4(Article for download at the end of the post).

Then on the different drive circuit is described based on the MOSFET drivers from Microchip, hence I am stopping here following this particular application note.

Now on I am using the following paper to find the various driver circuit :

"Design And Application Guide For High Speed MOSFET Gate Drive Circuits"

By Laszlo Balogh

Classification of  MOSFET drivers is as follows :

GATE REFERENCED DRIVERS

1. PWM direct drive
2. Bipolar totem pole driver
3. Mosfet totem pole driver
4. Speed enchancement circuits
5. Synchronous rectifier drive (Hard to acheive very fast,accurate and adaptive timing for this circuit                                                    by traditional design techniques)

HIGH SIDE NON ISOLATED GATE DRIVES

High side driver circuit for P channel devices

 P channel direct drive
 P channel level shifted drive

High side driver circuit for N channel devices

  N channel direct drive
  N channel bootstrap gate drive circuit
              Discrete high performance floating driver
              Integrated booster drivers

AC coupled gate drive circuits

Transformer coupled gate driver circuit

Single ended transformer coupled gate drive circuits

Double ended transformer coupled gate drives


DECISION REGARDING THE DRIVE CIRCUIT

Drive circuit for low side driving of the converter

The circuit used here will be a PNP turn off circuit that has speed enhancement and also will use the

dv/dt protection along with this gate drive circuit.

Drive circuit for high side driving of the converter

The drive circuit will be a bootstrap driver as shown is below figure.It says it is a integrated bootstrap driver in the application note but I do not know why it so.If someone knows please do add in comments.

Here is the figure provided by him in the above mentioned paper.

Drive circuit for low side switches of the Push-Pull transformer 

For low side switches of the Push-Pull transformer driver with enough current tolerance IRS2110 (2A rating  but max is around 2.5A ) could be used or IR 2010(with 3A rating-slower rise and fall time) would also be fine.

Please do correct me if i did anything or understood anything wrong.

Application Note from ST

Microchip application note

Different gate drive circuits