lecture (07) dc chopperdraelshafee.net/spring2018/epc403-power-electronics...theory of operations...

Lecture (07)

DC Chopper

By:

Dr. Ahmed ElShafee

Dr. Ahmed ElShafee, ACU : Spring 2018, EPC403 Power

Electronics 1

introduction

• DC choppers are used to convert DC with fixed voltage to controlled voltage DC.

• DC choppers are widely used in industrial applications like motors speed controllers.

• DC choppers, efficient and save a lot of energy.

Dr. Ahmed ElShafee, ACU : Spring 2018, EPC403 Power Electronics 2

Theory of operations

• DC chopper is simple a switch.

• When switch S is on for Ton, then become off for Toff

• The average output voltage on load R can be calculated as


• Source voltage

• On time (period)

• Off time (period)

• Total time

• Output voltage (average)

• Duty factor


• The average current

• Voltage root mean square value (effective voltage)

• Load dissipated (Po) power equals to source consumption power (Ps) (ideal chopper)


• Equation shows that output voltage can be controlled by chopper duty factor(k)

• Taking into consideration that chopping frequency is very high exceeds 500 hz to 2500 hz

• Switching or chopping device can be power transistor or IGBT, GTO,…etc

• Voltage drop over chopping device, is very small in range of 0.5 to 2 volts, to be ignored if output voltage is relativity high


• Vo in switching on period

• Vd is chopping device voltage drop during switching on period

• Vrms (effective voltage)

• Consumed power from supply


• Load consumed power


• Dc chopper can controlled the output DC voltage and consumed power by controlling duty factor

• Duty factor can in-turn controlled using

– PWM: by changing Ton (on period) while keeping T (total /duty period) constant

– FM: by changing duty period (T) while keeping Ton constant


• Chopper types (according to switching device)

– Thyristor chopper:

• Here thyristors are used as switch, used in high power/voltage loads, need external triggering/ off circuit

• BJT power transistor chopper, here device turned on by applying pulse on base, it will tun on at the bagaining of pulse and turn off by the end of the pulse. Not recommended as base current is increased by increasing load current .


– MOSFET chopper: has many advantages like

• Gate current is very small, as MOSFET turned on by field effect not current unlike BJT

• Gate is isolated from channel (drain/source) independent

• Works on high frequency (tiny switching time)

– dis-advantages:

• load power is lower than thyristors/and BGT


Step-down Chopper

• Here the agerave output voltage is smaller that source voltage where duty factor is less than 1

• Figure shows a stop down chopper using MOSFET that drives load of battery, resistor, and coil (inductive load)


• When MOSFET Q is on, current flow from source to load through MOSFET Q

• When MOSFET is in off mode, load current flow though free path diode till Q is on again

• Here the average output voltage is as

• Duty factor is less than 1


Example

• Step down chopper used to control output volatge that drives resistive load of 15 ohm. Source voltage is 300 volts. Chopping frequency is 1Khz and duty factor is 60%. Voltage drop on MOSFET = 2 volt.

• Calculate

– Duty period

– On/off period

– Average output voltage

– Effective RMS voltage

– Source power

– Load power

– Chopper efficiency

Dr. Ahmed ElShafee, ACU : Spring 2018, EPC403 Power Electronics

15


rms

Step UP DC Chopper

• Here the output load voltage is greater than the source voltage.

• Chopper as shown in figure consists of MSOFET, coil, and diod connected as shown


• When MOST Q is on, current flow through coil L , coil stores energy during on period.

• When chopper becomes off, coil will start dissipation power to the load through diode D

• Vo L output voltage

• ΔI: difference between highest and lowest current

• K: duty factor

• Vs: source voltage


Thanks,..

Dr. Ahmed ElShafee, ACU : Spring 2018, EPC403 Power Electronics

20

lecture (07) dc chopperdraelshafee.net/spring2018/epc403-power-electronics...theory of operations...

Documents