Ideal and Ideal and Practical Practical SwitchesSwitches

Dr.ZAH, UTHM 2

Ideal and Practical SwitchesTo assess the performance of a switch, we look at two aspects of its behaviour : (i) static, and

(ii) dynamic

If the switch is either in its ON or OFF state, we call this a static condition.

The dynamic condition is the transition from one static state to the other.

Dr.ZAH, UTHM 3

On-state : 1. Able to carry any value of forward current2. Has zero voltage drop3. Has zero on-state resistance4. Has zero power dissipation

Off-state: 1. Able to withstand infinite open-circuit voltage2. Has zero leakage current3. Has infinite off-state resistance

Ideal Dynamic Characteristics:

Ideal static characteristics :

1. Zero turn-on time2. Zero turn-off time3. Infinite dv/dt rating4. Infinite di/dt rating

Dr.ZAH, UTHM 4

Static performance of practical switches

• Power semiconductor switches depart to some extent from the ideal – there is a small but finite voltage drop in the ON state and a small but finite “leakage current” in the OFF state.

• The leakage current that flows in the OFF state causes the power dissipation in the device.

• Usually the power dissipation due to this leakage current is small in comparison with the power dissipation in the ON state therefore the power dissipation due to OFF state leakage can be neglected.

Dr.ZAH, UTHM 5

Dynamic performance of practical switchesA real switch needs a finite time ton for ON switching and toff for OFF switching.

These finite switching times have two major consequences: i. They limit the highest repetitive switching frequencies possible. ii. They introduce additional power dissipation in the switched

themselves.

Dr.ZAH, UTHM 6

Switching characteristics of practical devicesPractical switching devices have non-zero :i. rise timeii. fall timeiii. delay timeiv. storage time

i

t

t

tr tstftd

ton toff

t

tc

v

vcontrol

Turn‐on Turn‐offtd : delay timets : storage timetf : fall timetr : rise timeton : turn-on timetoff : turn-off timeton = td + trtoff = ts + tf

Voltage and current reference directions

vPower Switch

i

Dr.ZAH, UTHM 7

• During the transition there is power dissipation taking place inside the switch.

• Instantaneous power dissipated is given by the product of terminal voltage v and terminal current i.

Power Loss

i

t

tr tstftd

ton toff

t

tc

v Turn-on Turn-off

Turn-on loss Conduction

loss

Turn-off loss

t

Switching characteristics of practical devices (continued):

Voltage and current reference directions

vPower Switch

i

Dr.ZAH, UTHM 8

Power Loss Model of a Generic Switch

Pout = Pin + Pgate - Ploss

In general, the gate power input is very much smaller that that of the supply power, Pin . So, we can write

Pout = Pin - Ploss

Energy conservation principle requires that

Gate power input, Pgate

Useful output power, Pout

Input power, PinPower lost, Ploss

Dr.ZAH, UTHM 9

Switching LossesFour types of power losses occur in a practical switch:1. Turn-on loss2. Conduction loss3. Turn-off loss4. Gate driver power input

The instantaneous power loss in a practical switch is given by the expression

p(t) = v(t).i(t)

The average power loss during a time T is

T

dpT

P0

)(1

Dr.ZAH, UTHM 10

i. Average turn-on loss:

rt

ron dp

tP

0

)(1

ii. Average turn-off loss is

offt

offoff dp

tP

0

)(1

iii. Average conduction loss is

ondtc

condcond dp

tP

0

)(1 Power Loss

i

t

tr tstftd

ton toff

t

tc

v Turn-on Turn-off

Turn-on loss Conduction

loss

Turn-off loss

t

Three types of power losses in a practical switch: Voltage and current reference directions

vPower Switch

i

Dr.ZAH, UTHM 11

tcond offr t tt

s

dpdpdpT 0 00

)()()(1

where Ts is the switching period. The switching frequency is

ss T

f 1

The total energy W (in joules) dissipated in the switch in one switching cycle is given by the sum of the areas under the power waveform during ton and toff . Hence, the average power dissipation is

sdiss T

WP

Dr.ZAH, UTHM 12

It is important to consider losses in power switches:

i. to ensure that the system operates reliably under prescribed ambient conditions

ii. so that heat removal mechanism (e.g. heat sink, radiators, coolant) can be specified. Heat sinks and other heat removal systems are costly and bulky.

iii. losses in switches affects the system efficiency

If a power switch is not cooled to its specified junction temperature, the fullpower capability of the switch cannot be realised. Derating of the power switchratings may be necessary.

Dr.ZAH, UTHM 13

Consider a power switching device whose current and voltage waveforms are as shown in Figure 1. Determine(a) the turn-on energy loss (b) the turn-off energy loss(c) the average power dissipation.Assume a switching frequency of 40 kHz.

Example

250 V

100 A

1 μs 3 μst

Dr.ZAH, UTHM 14

Practical switch specificationsImportant parameters of practical devices to consider when usingthem as a switch:

1. Voltage ratings: forward and reverse repetitive peak voltages, and ON-state forward voltage drop.

2. Current ratings: average, rms, repetitive peaks, non- repetitive peak, OFF-state leakage current.

3. Switching frequency/switching speed: Device heatingincreases with the switching speed.

4. di/dt rating: The power switching device needs a minimumamount of time before its whole conducting surface comes into play in carrying the full current. If the current rises rapidly, the current flowing may be concentrated to a certain area and the device may be damaged.

Dr.ZAH, UTHM 15

Practical switch specifications (continued)

5. dv/dt rating: A semiconductor device has an internal junction capacitance, Cj . If the voltage across the switch changes rapidly during turn-on, turn-off, and while connecting the main supply, the current Cj dv/dt flowing through Cj may be too high, thereby causing damage to the device.

6. Safe operating area: sets the maximum current, voltage, and power loss that can be handled safely by the device.

7. Temperature: maximum allowable junction temperature.