MOSFET in Power supply Application

MOSFET & It's Application in SMPS : 

Jaydeep Shah (06- sep 2019 )

MOSFETs are available in two basic forms:

• Depletion Type: The transistor requires the Gate-Source voltage (V_GS) to switch the device “OFF”. The depletion mode MOSFET is equivalent to a “Normally Closed” switch.
• Enhancement Type: The transistor requires a Gate-Source voltage(V_GS) to switch the device “ON”. The enhancement mode MOSFET is equivalent to a “Normally Open” switch.

Depending upon the type of materials used in the construction, and the type of operation, the MOSFETs are classified as in the following figure.

The N-channel MOSFETs are simply called as NMOS. The symbols for N-channel MOSFET are as given below.

The P-channel MOSFETs are simply called as PMOS. The symbols for P-channel MOSFET are as given below

 A lightly doped P-type substrate is taken into which two heavily doped N-type regions are diffused, which act as source and drain. Between these two N+ regions, there occurs diffusion to form an Nchannel, connecting drain and source. (Diffusion is net movement of anything from a region of higher concentration to a region of lower concentration.)

A thin layer of Silicon dioxide (SiO₂) is grown over the entire surface and holes are made to draw ohmic contacts for drain and source terminals.

It is a four-terminal device with source(S), gate (G), drain (D) and body (B) terminals. The body is frequently connected to the source terminal, reducing the terminals to three. It works by varying the width of a channel along which charge carriers flow (electrons or holes).

The charge carriers enter the channel at source and exit via the drain. The width of the channel is controlled by the voltage on an electrode is called gate which is located between source and drain.

In general, any MOSFET is seen to exhibit three operating regions

1. Cut-Off Region
   Cut-off region is a region in which the MOSFET will be OFF as there will be no current flow through it. In this region, MOSFET behaves like an open switch and is thus used when they are required to function as electronic switches.
2. Ohmic or Linear Region
   Ohmic or linear region is a region where in the current I_DS increases with an increase in the value of V_DS. When MOSFETs are made to operate in this region, they can be used as amplifiers.
3. Saturation Region
   In saturation region, the MOSFETs have their I_DS constant in spite of an increase in V_DS and occurs once V_DS exceeds the value of pinch-off voltage V_P. Under this condition, the device will act like a closed switch through which a saturated value of I_DS flows. As a result, this operating region is chosen whenever MOSFETs are required to perform switching operations.

Figure 1a shows the transfer characteristics (drain-to-source current I_DS versus gate-to-source voltage V_GS) of n-channel Enhancement-type MOSFETs. From this, it is evident that the current through the device will be zero until the V_GS exceeds the value of threshold voltage V_T. This is because under this state, the device will be void of channel which will be connecting the drain and the source terminals. Under this condition, even an increase in V_DS will result in no current flow as indicated by the corresponding output characteristics (I_DS versus V_DS) shown by Figure 1b. As a result this state represents nothing but the cut-off region of MOSFET’s operation.

Next, once V_GS crosses V_T, the current through the device increases with an increase in I_DS initially (Ohmic region) and then saturates to a value as determined by the V_GS (saturation region of operation) i.e. as V_GS increases, even the saturation current flowing through the device also increases. This is evident by Figure 1b where I_DSS2 is greater than I_DSS1 as V_GS2 > V_GS1, I_DSS3 is greater than I_DSS2 as V_GS3 > V_GS2, so on and so forth. Further, Figure 1b also shows the locus of pinch-off voltage (black discontinuous curve), from which V_P is seen to increase with an increase in V_GS.

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·         SELECTION OF THE MOSFET :  

Voltage and Current Ratings

The drain-source voltage VDSS rating is important in selecting MOSFETs. Application of a voltage exceeding VDSS might result in the destruction of a MOSFET. It is necessary to choose MOSFETs with a VDSS sufficiently higher than the voltage at which they will actually be used. However, MOSFETs with high VDSS ratings tend to have large on-state resistance, RDS(ON). A downside of using such MOSFETs is increased conduction loss. If you select MOSFETs according to possible peak surge voltage, you could end up opting for devices with large on-state resistance. To address this situation. Generally, on-state resistance determines the upper limit of the drain current ID. Ensure that not only the loss calculated as ID2×RDS(ON), the permissible power dissipation but also a temperature rise due to heating does not cause the device to exceed its operating temperature range.

Switching Speed

When a power MOSFET switches at a high frequency, its switching loss accounts for a significant portion of total loss. To reduce total loss for high frequency switching applications, high speed power MOSFETs should be used.

What is MOSFET Switching Time?

The MOSFET will turn ON or OFF after the Gate voltage turns ON/OFF. The time in between turning ON or OFF is called the switching time.

 EX:

V_GS threshold: V_{GS(th) :}

V_GS(th) is the voltage required between the Gate and Source to turn ON the MOSFET.In other words, supplying a voltage greater than V_GS(th) will turn ON the MOSFET.
To determine the amount of current that flows through the MOSFET when ON it is necessary to refer to the specifications and electrical characteristics for each element.

Table 1 lists the relevant electrical characteristics. In the case of V_DS=10V a threshold voltage of between 1.0V and 2.5V is required for an I_D of 1mA.

Parasitic Capacitance

Parasitic capacitance exists in power MOSFETs as shown in Figure 1.

Sometimes known as stray capacitance, parasitic capacitance is unavoidable and typically unwanted that exists between the parts of an electronic component or circuit simply because of how close they are to one another. Capacitance is the ability of a system to store an electric charge.

The Gate terminal in a MOSFET is isolated from the other terminals by an oxide film. The silicon under the gate has the opposite polarity to the drain and source which results in the formation of PN junctions (diode) between the Gate, Drain and Source regions. Cgs and Cgd are the capacitance of the oxide layers, while Cds is determined by the junction capacitance of the internal diode.

Switching Time and Drive Conditions :

Bipolar transistors need a large base current to maintain low on-state voltage. In contrast, since power MOSFETs are voltage-controlled devices, they can be driven just by charging gate capacity, and are therefore a low in power consumption.

Note, however, that power MOSFETs have a slightly large input capacitance Ciss. Thus, for high speed switching applications, it is necessary to quickly charge the input capacitance from a low-impedance signal source.

Low-impedance drive is required to reduce turn-on time. However, the use of a high gate voltage results in the much charging of gate-source capacitance, resulting in an increase in td (off).

Switching time can be controlled via gate resistance. If you want to change the turn-on and turn-off switching speeds separately, you can use diodes to change the gate resistance values for turn-on and turn-off. Figure 2.3 shows some examples.

Gate Drive Resistor

RG is the gate driver resistor for the power switch, MOSFET. The selection of this resistor value must be done in conjunction with EMI compliance testing and efficiency testing. Using a larger resistor value for RG slows down the turnon and turnoff of the MOSFET. A slower switching speed reduces EMI but also increases the switching loss. A trade-off between switching loss and EMI performance must be carefully performed.

·         LOSSES IN MOSFET :

In the ideal switching regulator shown in Figure 2, the current is zero when the switch is open and the power loss is zero, thus VIN is being chopped. When the switch is closed, the voltage across it is zero and the power loss is also zero. An ideal switch implies zero losses, thus offering 100% efficiency. An efficient switching regulator results in less heat dissipation, which reduces system cost and size for elements such as heat sinks, fans and their assembly. In batteryoperated systems, less power loss means that these devices can use the same battery for a longer run time because the device pulls less current from the battery.

MOSFETs have a finite switching time, therefore, switching losses come from the dynamic voltages and currents the MOSFETs must handle during the time it takes to turn on or of, Gate-drive losses are also switching losses because they are required to turn the FETs on and off.

Conduction loss : RDS(on) * I(sw)-(RMS)

MOSFET switching losses are a function of load current and the power supply’s switching frequency as shown by below Equation

Psw = Vin * Iout * Fsw * ((Qgs + Qgd) / Ig )

where VIN = VDS (drain-to-source voltage),

IOUT = ID (drain current),

fSW is the switching frequency,

QGS2 and QGD depend on the time the driver takes to charge the FET

 IG is the gate current.

 Switch-MOSFET gate losses can be caused by the energy required to charge the MOSFET gate. That is, the QG(TOT) at the gate voltage of the circuit. These are both turn-on and turn-off gate losses. Most of the power is in the MOSFET gate driver. Gatedrive losses are frequency dependent and are also a function of the gate capacitance of the MOSFETs. When turning the MOSFET on and off, the higher the switching frequency, the higher the gate-drive losses. This is another reason why efficiency goes down as the switching frequency goes up.Larger MOSFETs with lower RDS(on) provide lower conduction losses at the cost of higher gate capacitances, which results in higher gate-drive losses.

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Conclusion :

Power loss in a MOSFET comes from two sources. Every MOSFET has a resistive element, so it dissipates power as current is conducted through the device. The resistive parameter is described as on-resistance, or R_DS(ON). These conduction losses are inversely proportional to the size of the MOSFET; the larger the switching transistor, the lower its R_DS(ON) and, therefore, its conduction loss.

The other source of power loss is through switching losses. As the MOSFET switches on and off, its intrinsic parasitic capacitance stores and then dissipates energy during each switching transition. The losses are proportional to the switching frequency and the values of the parasitic capacitances. As the physical size of the MOSFET increases, its capacitance also increases; so, increasing MOSFET size also increases switching loss. 

These sources of power loss create a significant challenge for power supply designers. While a larger MOSFET will exhibit less on-resistance – and consequently lower conduction loss – its larger area drives up parasitic capacitance and switching loss. In many cases, moving to a larger MOSFET to reduce conduction loss will result in so much increased switching loss that it outweighs the conduction loss savings. Designers typically try to achieve a balance between conductive and switching losses for a particular application.

FAQ :

1) Is it possible to use the Zener diode between the gate and source for surge absorption?

The Zener diode between gate and source is intended for electrostatic breakdown protection. Please confirm whether overvoltage is applied to the gate in the actual state.

2) What does the dv/dt of the MOSFET mean?

The dv/dt of the MOSFET is the changing rate of the drain-source voltage during the switching transient.

If dv/dt is too large, ringing may occur, possibly leading to MOSFET damage.

Therefore, dv/dt ruggedness values are specified for some MOSFETs.

3) How does the series gate resistor affect the MOSFET?

When using lower gate resistance, switching time becomes shorter, and ringing (damped oscillation) may occur.
Ringing can cause oscillation and EMI noise.
When using higher gate resistance, switching time becomes longer.
As a result, switching loss increases and heat is generated.
In the bridge circuit, a short circuit may occur across the upper and lower MOSFETs by combination of the gate resistances.
Therefore, it is necessary to consider the optimum gate resistance.

4) The MOSFET does not turn off by a turn-off signal. How do I solve this problem?

Confirm whether the control signal voltage V_drive is close to the GND or not.
When the voltage is not close to the GND, the output impedance of the gate drive circuit may be high. Consider lowering output impedance. (Select different turn off path for turning off the MOSFET quickly).
Confirm whether the gate-source voltage V_GS is the same as the control signal or not. If it is not, insert a pull-down resistor R_pull between gate and source to ensure the gate-source voltage V_GS to the GND level.
Please select R_pull accordinglly that does not affect the gate driving performance.

5) Is the on-state resistance of a MOSFET dependent on temperature?

The MOSFET has a positive temperature coefficient. Since values of coefficients depend on the drain-source breakdown voltages and the process, it is necessary to confirm with the data sheet etc. When performing thermal design / circuit design, please consider this temperature fluctuation.

6) Is the MOSFET drive current necessary?

For bipolar transistors, a large base current is required to maintain low on-voltage. However, since the MOSFET is a voltage control element, it can drive with small power sufficient to charge the gate. But because the input capacitance (Ciss) of the power MOSFET is rather large for high-speed switching, it is necessary to quickly charge the input capacitance with a low-impedance drive circuit.

For an indication of the gate current required for charging, it is possible to calculate the drive current simply by using the Qg characteristic prescribed on the data sheet. The required switching time is “t (desired)” (for example t_on or t_off), which becomes the following expression.
Ig=Qg/t(desired)

7) Are there any special considerations for using MOSFETs?

For MOSFETs, the maximum allowable current, voltage, power dissipation and other characteristics are specified as maximum ratings. Maximum ratings are the highest absolute values that must not be exceeded even instantaneously under any conditions. A device may not be able to recover from stress that exceeds a specified maximum rating. None of the absolute maximum ratings may be exceeded. Care should therefore be exercised regarding bounces in supply voltage, variations in the characteristics of electronic devices, possible exposure to stress higher than maximum ratings during circuit adjustment, changes in ambient temperature, fluctuations in the input signal, and other such factors. Please select the appropriate device according to the application.
Also, even within the absolute maximum ratings and the operating range, reliability will change significantly depending on the degree of derating

8)  Electrical characteristics of MOSFETs (Dynamic Characteristics　t_r/t_on/t_f/t_off)

Switching characteristics

Since power MOSFETs are majority-carrier devices, they are faster and capable of switching at higher frequencies than bipolar transistors.
Switching time measurement circuit and input / output waveform are shown below.

1.      t_{d (on)}: Turn-on delay time
The time from when the gate-source voltage rises above 10% of V_GS until the drain-source voltage reaches 90% of V_DS

2.      t_r：Rise time
The time taken for the drain-source voltage to fall from 90% to 10% of V_DS

3.      t_on：Turn-on time
The turn-on time is equal to t_{d (on)}+ t_r.

4.      t_{d (off)}：Turn-off delay time
The time from when the gate-source voltage drops below 90% of V_GS until the drain-source voltage reaches 10% of V_DS

5.      t_f：Fall time
The time taken for the drain-source voltage to rise from 10% to 90% of V_DS

6.      t_off：Turn-off time
The turn-off time is equal to t_d(off)+ t_f.

Characteristics		Symbol	Min	Typ	Max	Unit
Switching time	Rise time	tr	—	13	—	ns
	Turn-on time	ton	—	26	—
	Fall time	tf	—	14	—
	Turn-off time	toff	—	63	—

9) Electrical characteristics of MOSFETs (Charge Characteristic Q_g/Q_gs1/Q_gd/Q_SW/Q_OSS)

Gate charge

Because the Gate (G) input terminal of a MOSFET is insulated, the amounts of charge Q seen from the Gate are important characteristics. Figure 1.5 illustrates the definitions of gate charge characteristics.

Total gate charge Q_g

The amount of charge to apply voltage (from zero to designated voltage) to gate

Gate-source charge 1 Q_gs1

The amount of charge required for a MOSFET to begin to turn on (before dropping drain-source voltage)

Gate-drain charge Q_gd

The amount of gate charge charged in the Miller plateau

Gate switch charge Q_sw

The amount of charge stored in the gate capacitance from when the gate-source voltage has reached V_th Until the end of the Miller plateau

Output charge Q_oss

Drain-source charge

The definition of the gate charge amount is shown in the figure below.

Data sheet description

Characteristics	Symbol	Test Conditions	Min	Typ	Max	Unit
Total gate charge	Qg	VDD ≈ 20 V, VGS = 10 V, ID = 50 A	—	103	—	nC
		VDD ≈ 20 V, VGS = 4.5 V, ID = 50 A	—	49	—
Gate-source charge 1	Qgs1	VDD ≈ 20 V, VGS = 5 V, ID = 50 A	—	25	—
Gate-drain charge	Qgd		—	12.4	—
Gate switch charge	QSW		—	23	—
Output charge	QOSS	VDS = 20 V, VGS = 0 V	—	85.4	—

9) Electrical characteristics of MOSFETs (Static Characteristics R_DS(ON))

Drain-source on-state resistance (R_DS(ON)）

The resistance across drain and source when the MOSFET is in the "on" state

V_DS(ON)measurement


The specified constant drain current, I_D, is applied until V_GSreaches the specified voltage. At this point, drain-source voltage is measured. On-state resistance is calculated by dividing it by the value of drain current, I_D.
Note: Keep the open-circuit voltage of the constant-current source below the drain-source breakdown voltage.

Measurement of forward transfer admittance

Gate-source voltage, V_GS, is increased until drain current, I_D, reaches the specified value. Then, V_GSis changed only slightly, and the resulting change in drain current, I_D, is measured.

Data sheet description

Characteristics	Symbol	Test Conditions	Min	Typ	Max	Unit
Drain-source on-state resistance	RDS(ON)	VGS = 4.5 V, ID = 50 A	—	0.95	1.35	mΩ
		VGS = 10 V, ID = 50 A	—	0.65	0.80

10) Electrical characteristics of MOSFETs (Static Characteristics V_th)

Gate threshold voltage (V_th)

V_th stands for "threshold voltage." V_th is the gate voltage that appears when the specified current flows between source and drain.

V_th measurement :

Gate-source voltage, V_GS, is increased until drain current, I_D, reaches the specified value, at which point V_GSis measured.

Data sheet description

Characteristics	Symbol	Test Conditions	Min	Typ	Max	Unit
Gate threshold voltage	Vth	VDS = 10 V, ID = 1.0 mA	1.4	—	2.4	V

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Author:

 JAYDEEP SHAH (E.C )

AHMEDABADN – INDIA

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