EE 603 Power Electronics

 

Homework # 6          Due March 1, 2006

 

Read the about non-ideal diode effects. Read the UC1526 PWM chip specification, the International Rectifier HFA04TB60 4A, 600V ultrafast recovery diode specification, and the Texas Instruments paper on step down converters. All of these are on the EE603 web site at www.engr.uky.edu/~radun under EE603 Power Electronics / Copies of selected manufacturer device specifications.

 

Problem 1

 

Print out a copy of the block diagram of the UC1526 PWM chip. It is available in pdf format from the EE603 web site under “Copies of selected manufacturer device specifications.”

 

a)     Indicate (by circling and labeling) the oscillator section, the PWM comparator, and the single switching cycle per oscillator flip-flop.

b)     What are the peak and valley values of the ramp wave?

c)      Choose oscillator program resistor and capacitor values to achieve a 100kHz ramp wave output.

d)     Choose external component values for the amplifier indicating how they connect to the chip (connect them to the correct pin numbers on the chip), so 5V in gives a duty cycle equal to 0.5.

e)     The UC1526 has two outputs while the step down converter needs a chip with one output. What does the second flip-flop do and how are the two UC1526 outputs related to the PWM comparator output? You will need to go through the logic on the chip making a timing diagram for the logic.

f)       Combine the two UC1526 outputs (design a simple logic circuit (use logic gates) to combine them) to obtain a single output suitable for the step down converter.

 

 

Problem 2

 

Shown below are three different comparator / amplifier circuits that can be simulated using B2 SPICE. Figure 1 shows a simplified BJT circuit with current sources providing the bias. Figure 2 shows a more complete CMOS circuit with biasing provided using current mirrors and a current source reference created from a depletion mode MOSFET (MOS 8). Note that the 10meg W resistor in Fig. 2 is not required in practice. It is required for B2 SPICE to connect the circuit correctly. There may be a bug in B2 SPICE. Figure 3 shows an amplifier built into B2 SPICE. I used the operational amplifier in the more devices menu. You may try running all three circuits. Try the B2 SPICE circuit at 25kHz. Explain what you have to do to get the circuit to work.

Figure 1  Simplified BJT comparator / amplifier. NPN transistor parameters are bf = 110, Is = 0.9 10-16, cje = 1 10-14, cjc = 1 10-14. PNP transistor parameters are bf = 110, Is = 0.9 10-16, cje = 1 10-14, cjc = 1 10-14. Other Spice parameters are their default values.

Figure 2  MOS comparator / amplifier. P channel MOSFETs 0, 3 and 10 have parameters vt = -1, kp = 2.263 10-4, lambda = 0.02, cbd = 1 10-15, cbs = 1 10-15, is = 1 10-14, and tox = 1 10-7. P channel MOSFETs 1 and 2 have parameters vt = -1, kp = 1.6 10-4, lambda = 0.02, cbd = 1 10-15, cbs = 1 10-15, is = 1 10-12, and tox = 1 10-7. N channel MOSFETs 4, 5, 6, 7 and 9 have parameters vt = 1, kp = 5 10-5, lambda = 0.02, cbd = 1 10-14, cbs = 1 10-14, and tox = 1 10-7. N channel MOSFETs 8 has parameters vt = -5, kp = 2.5 10-5, lambda = 0.02, cbd = 1 10-14, cbs = 1 10-14, and tox = 1 10-7. Other Spice parameters are their default values.

Figure 3  B2 SPICE base comparator / amplifier.

 

a)     For each of the circuits in Fig 1 and Fig. 2 label the plus and the minus inputs.

b)     What function do the diodes in Fig. 1 perform and why aren't they needed in the circuit in Fig. 2?

c)      Simulate the circuit in Fig. 2 with a 1kHz triangle wave going from 1V to 14V with a 15V Vcc for Vref = 2VDC, 6VDC, 10VDC, and 12VDC. Plot your result for three cycles and Vref = 2VDC and 12VDC. You can use the pulse waveform with a slow rise time to create the saw tooth waveform.

d)     What is the tceiling that you used?

e)     Try the other circuits but you do not have to hand in the results. Try the B2 SPICE circuit at 25kHz. Explain what you have to do to get the circuit to work. Again you do not have to turn the results in.

 

 

Problem 3

 

Junction Capacitance

 

a)     Why is a capacitor’s current zero when its voltage is constant at say 0.7V?

b)     Using Q = C V, prove the circuit equation  if C is a constant. What is the circuit equation if the capacitance is not a constant but rather depends on the capacitor voltage (C = C(VC))?

c)      If Q = f(V) what is C?

d)     How do you compute the diode’s junction capacitance from the measured current, the AC voltage, and the frequency? Why doesn’t the DC voltage enter into the calculation?

e)     Build a SPICE model of the circuit in Fig. 1 for a diode with parameters IS = 9 10-14A, n =1, CJO = 23pF, VJ = 0.7V, m = 0.336 and TT=5ms.

f)       The source consists of a DC part and an AC part, which in Spice can be created with a single transient source. Make the AC part of the source 1V P-P at 100kHz. What should your step size be? When set up your simulation (set the time step and duration) make sure you do not check “use initial conditions” so Spice will compute them. This way your simulation immediately reaches steady state.

g)     When you observe the diode voltage, it will have both an AC and a DC part. Only the AC part is of interest. The DC part can be removed using a coupling capacitor and a resistor as shown below. Why are they connected as shown? Assume the resistance is 1megW. How do you compute the Ccouple value? Verify that 200pF meets your requirement.

h)     Simulate and measure the capacitance of the diode for VDC = 1V, 2V, and 4V. Thus, make the DC part of the Spice transient source equal to 1V, 2V, and 4V. Run your simulation for three cycles. Plot VAC and IR for 2V DC reverse bias voltage. You may normalize one of them so they both fit on the same plot.

i)        Compute the diode’s junction capacitance for DC reverse bias voltages equal to 1V, 2V, and 4V using Eq. 1 in the non-ideal diode effects notes on the web. Compare your “measured results” to those obtained with Eq. 1. Try to explain any discrepancies you observe.

 

Fig. 1

Reverse recovery

 

i)        Make a Spice model of the circuit in Fig. 2 using Rsense = 10kW, and Vin equal to a 20Vp-p (goes from -10V to +10V) square wave at 10kHz. Use the Spice pulse input with the rise and fall times equal to 1ms. Compute and plot the voltage across the sense resistor with the diode parameters in part four first making TT=0s and then making TT = 5ms. What should your time step be? Plot the voltage Vsense for three cycles. What do you observe?

 

 Fig. 2

 

j)         What is the International Rectifier HFA04TB60 4A, 600V ultrafast recovery diode’s reverse recovery time and its Cjo.