1

EE462L, Fall 2011Diode Bridge Rectifier (DBR)

2

120/25VTransformer

120VVariac

Important − never connect a DBR directly to 120Vac or directly to a variac. Use a 120/25V

transformer. Otherwise, you may overvoltage the electrolytic capacitor

Diode Bridge Rectifier(DBR)

+

–

+≈ 28Vac rms

–

1

4

3

2

Equivalent DC load resistance RL

+≈ 28√2Vdc ≈ 40Vdc−

Iac

Idc

Be extra careful that you observe the polarity markings on the electrolytic capacitor

3

Variac, Transformer, DBR Hookup

The 120/25V transformer has separate input and output windings, so the input voltage reference is not passed through to the output (i.e., the output voltage is isolated)

The variac is a one-winding transformer, with a variable output tap. The output voltage reference is the same as the input voltage reference (i.e., the output voltage is not isolated).

4

Example of Assumed State Analysis

+

Vac–

•Consider the Vac > 0 case

•We make an intelligent guess that I is flowing out of the source + node.

• If current is flowing, then the diode must be “on”

•We see that KVL (Vac = I • RL ) is satisfied

RL

•Thus, our assumed state is correct

+

–

5

Example of Assumed State Analysis

+10V

–

• We make an intelligent guess that I is flowing out of the 11V source

• If current is flowing, then the top diode must be “on”

RL

+

11V

–

+11V

–

•The bottom node of the load resistor is connected to the source reference, so there is a current path back to the 11V source

•KVL dictates that the load resistor has 11V across it

− 1V +

•The bottom diode is reverse biased, and thus confirmed to be “off”

•Current cannot flow backward through the bottom diode, so it must be “off”

•Thus our assumed state is correct

Auctioneering circuit

6

Assumed State Analysis

• Consider the Vac > 0 case

+

–

+Vac–

1

4

3

2RL

What are the states of the diodes – on or off?

• We make an intelligent guess that I is flowing out of the source + node.

• I cannot flow into diode #3, so diode #3 must be “off.” I flows through RL.

• I comes to the junction of diodes #2 and #4. We have already determined that diode #4 is “off.” If current is flowing, then diode #2 must be “on,” and I continues to the –Vac terminal.

• I cannot flow into diode #4, so diode #4 must be “off.” If current is flowing, then diode #1 must be “on.”

7

Assumed State Analysis, cont.

+Vac > 0

–

1

2RL

• A check of voltages confirms that diode #4 is indeed reverse biased as we have assumed

• We see that KVL (Vac = I • RL ) is satisfied

• Thus, our assumed states are correct

+−

+

−

• A check of voltages confirms that diode #3 is indeed reverse biased as we have assumed

+−

• The same process can be repeated for Vac < 0, where it can be seen that diodes #3 and #4 are “on,” and diodes #1 and #2 are “off”

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AC and DC Waveforms for a Resistive Load

Vac

-40

-20

0

20

40

0.00 8.33 16.67 25.00 33.33

Milliseconds

Vol

ts

With a resistive load, the ac and dc current waveforms have the same waveshapes as Vac and Vdc shown above

+ Vac > 0

–

1

2

+ Vdc

– – Vac < 0

+

4

3 + Vdc

–

Vdc

-40

-20

0

20

40

0.00 8.33 16.67 25.00 33.33

MillisecondsV

olts

Note – DC does not mean constant!

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EE362L_Diode_Bridge_Rectifier.xls

F - Hz C - uF VAC P - W 60 18000 28 200

05

101520

2530

35

4045

0.00 2.78 5.56 8.33 11.11 13.89 16.67

Milliseconds

Volts Vsource

Vcap

Peak-to-peak ripple voltage

C charges C discharges to load

Diode bridge conducting. AC system replenishing capacitor energy.

Diode bridge off. Capacitor discharging into load.

From the power grid point of view, this load is not a “good citizen.” It draws power in big gulps.

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DC-Side Voltage and Current for Two Different Load Levels

800W Load

200W Load

fT

1

Vdc

Idc

Ripple voltage increases

Average current increases (current pulse gets taller and wider)

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Approximate Formula for DC Ripple Voltage

CtPVVpeak

22min

2

CtPVVVV peakpeak

2))(( minmin

)(2)(

minmin VVC

tPVVpeak

peak

tPCVCVpeak 2min

221

21

Energy given up by capacitor as its voltage drops from Vpeak to Vmin

Energy consumed by constant load power P during the same time interval

12

Approximate Formula for DC Ripple Voltage, cont.

)(2)(

minmin VVC

tPVVpeak

peak

fT 1

peakripplepeaktopeakpeak fCV

PVVV2

)( min

, 2min peakpeak VVV For low ripple,

2Tt and

Δt

T/2

13

AC Current Waveform

fT 1=

14

Schematic

+

–

+ Vac

–

1

4

3

2

16-18mF Iac 2kΩ, 2W

discharge resistor 0.01Ω input current

sensing resistor

Iac

120/25V Transformer

120V Variac Idc

+ Vdc

–

Red wire

0.01Ω output current sensing resistor

LED

3.3kΩ, 1W

Notch or + sign

+

15

Mounting the Toggle Switch

Space left between hex nut and body of switch

16

Careful!

17

Thermistor Characteristics

Thermistor Resistance

0

0.5

1

1.5

2

2.5

3

3.5

0 10 20 30 40 50 60 70 80 90 100 110

Temp - deg C

Ohm

s - p

u

For our thermistor, 1pu = 1kΩ

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Solar Panel Temp

y = 15.557x3 - 51.869x2 + 95.653x - 25.875

-20

-10

0

10

20

30

40

50

60

70

80

0 0.5 1 1.5 2

Voltage

Tem

p - D

egre

es C

• Thermistor in series with 470Ω resistor

• Series combination energized by 2.5Vdc

• The voltage across the 470Ω resistor then changes with temperature as shown below

470Ω

Rtherm

2.5V

To data logger

As the thermistor gets hotter, more of the 2.5V appears across the 470Ω resistor

Excel curve fit. Coefficients entered into data logger.

Thermistor Resistance

0

0.5

1

1.5

2

2.5

3

3.5

0 10 20 30 40 50 60 70 80 90 100 110

Temp - deg C

Ohm

s - p

u

Measuring the temperature on the backside of a solar panel

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Measuring Diode Losses with an Oscilloscope

Tcond

condavgavg

Hz

condavgavgavg TIV

T

TIVP 240

4

60 Watts.

1

4

3

2

Scope probe

Scope alligator clip

Estimate on oscilloscope the average value Iavg of ac current over conduction interval Tcond

Estimate on oscilloscope the average value Vavg of diode forward voltage drop over conduction interval Tcond

Since the forward voltage on the diode is approximately constant during the conduction interval, the energy absorbed by the diode during the conduction interval is approximately V avg • I avg • Tcond . Each diode has one conduction interval per 60Hz period, so the average power absorbed by all four diodes is then

i(t)

v(t)

20

Forward Voltage on One Diode

Zero

Conducting

Zoom-In

Zero

Forward voltage on one diode

Forward voltage on one diode

21

AC Current Waveform

One pulse like this passes through each diode, once per cycle of 60Hz

The shape is nearly triangular, so the average value is approximately one-half the peak

View this by connecting the oscilloscope probe directly across the barrel of the 0.01Ω current-sensing resistor