MDC3105

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8

Using TTR Designing for Pulsed Operation

For a repetitive pulse operating condition, time averaging

allows one to increase a device’s peak power dissipation

rating above the average rating by dividing by the duty cycle

of the repetitive pulse train. Thus, a continuous rating of 200

mW of dissipation is increased to 1.0 W peak for a 20% duty

cycle pulse train. However, this only holds true for pulse

widths which are short compared to the thermal time

constant of the semiconductor device to which they are

applied.

For pulse widths which are significant compared to the

thermal time constant of the device, the peak operating

condition begins to look more like a continuous duty

operating condition over the time duration of the pulse. In

these cases, the peak power dissipation rating cannot be

merely time averaged by dividing the continuous power

rating by the duty cycle of the pulse train. Instead, the

average power rating can only be scaled up a reduced

amount in accordance with the device’s transient thermal

response, so that the device’s max junction temperature is

not exceeded.

Figure 12 of the MDC3105 data sheet plots its transient

thermal resistance, r(t) as a function of pulse width in ms for

various pulse train duty cycles as well as for a single pulse

and illustrates this effect. For short pulse widths near the left

side of the chart, r(t), the factor, by which the continuous

duty thermal resistance is multiplied to determine how much

the peak power rating can be increased above the average

power rating, approaches the duty cycle of the pulse train,

which is the expected value. However, as the pulse width is

increased, that factor eventually approaches 1.0 for all duty

cycles indicating that the pulse width is sufficiently long to

appear as a continuous duty condition to this device. For the

MDC3105LT1, this pulse width is about 100 seconds. At

this and larger pulse widths, the peak power dissipation

capability is the same as the continuous duty power

capability.

To use Figure 12 to determine the peak power rating for

a specific application, enter the chart with the worst case

pulse condition, that is the max pulse width and max duty

cycle and determine the worst case r(t) for your application.

Then calculate the peak power dissipation allowed by using

the equation,

Thus for a 20% duty cycle and a PW = 40 ms, Figure 12

yields r(t) = 0.3 and when entered in the above equation, the

Also note that these calculations assume a rectangular

pulse shape for which the rise and fall times are insignificant

compared to the pulse width. If this is not the case in a

be multiplied together and the resulting power waveform

integrated to find the total dissipation across the device. This

then would be the number that has to be less than or equal

to the Pd(pk) calculated above. A circuit simulator having a

waveform calculator may prove very useful for this purpose.

Notes on SOA and Time Constant Limitations

Figure 10 is the Safe Operating Area (SOA) for the

MDC3105. Device instantaneous operation should never be

pushed beyond these limits. It shows the SOA for the

Transistor “ON” condition as well as the SOA for the Zener

during the turn−off transient. The max current is limited by

the Izpk capability of the Zener as well as the transistor in

addition to the max input current through the resistor. It

should not be exceeded at any temperature. The BJT power

dissipation limits are shown for various pulse widths and

duty cycles at an ambient temperature of 25

°C. The voltage

the input to the device is switched off, the BJT “ON” current

is instantaneously dumped into the Zener diode where it

begins its exponential decay. The Zener clamp voltage is a

function of that BJT current level as can be seen by the

addition to the Zener’s current limit impacting this device’s

500 mA max rating, the clamping diode also has a peak

energy limit as well. This energy limit was measured using

a rectangular pulse and then translated to an exponential

equivalent using the 2:1 relationship between the L/R time

constant of an exponential pulse and the pulse width of a

rectangular pulse having equal energy content. These L/R

load should not exceed these limits at their respective

currents. Precise L/R limits on Zener energy at intermediate

current levels can be obtained from Figure 11.