### Current Limiting Diodes (part 3)

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Summary: Current Limiting Diodes, also called Current Regulating Diodes or Constant Current Diodes, are devices that act like a dual of a zener diode. That is, over a range of applied voltage they will exhibit a constant current characteristic. Refer to the previous articles on Current Limiting Diodes (Current Limiting Diodes and Current Limiting Diodes (part2):

This concluding article will discuss some of the uses and performance for CLD devices.

Extending CLD Device Performance:

One problem in using CLD devices (and zener/avalanche diodes for that matter) is the granularity and range of the allowable device currents (voltages). There are simple circuits that enable the current of a given CLD device to be increased. One is shown in Figure 1 following:

Figure 1
NPN CLD current booster

The schematic is taken from an article entitled "Boosting the Current Limit of Current Limiting Diodes" by Sze Chin, Central Semiconductor Corp. A link to this may be found at the bottom of the page:
http://www.centralsemi.com/engineering/index.aspx

There is a little derivation wherein it is shown that ideally the boosted current is approximately the CLD current multiplier by the ratio of R2/R1 plus 1. This is somewhat optimistic, as it is based on a perfect transistor.

A graph of the circuit output using a 2n3055 and R2 = 200 ohms is shown in Figure 2 following:

Figure 2
NPN CLD current booster graph

In Figure 2 a family of curves was plotted with R2 = 200 ohms and R1 taking on the values shown. The sweep is 100V in 100 mSec, so the abscissa is equal to 1V per millisecond. If we perform a little more detailed derivation, using a value for Vbe and for the current gain we can arrive at a better formula for the current gain to be expected.

This is not a bad way to multiply CLD current, nonetheless, even though the outputs vary a bit when R1 is a small value. Looking at the trace for R1 = 8 ohms, we see that the current changes by about 40ma over the input voltage (regulation) range of about 80 volts, much less than would occur for a series resistor implementation.

But now might this affect the AC noise rejection of the circuit?

A test circuit was created as shown in Figure 3 following:

Figure 3
NPN CLD current booster test circuit

Figure 4
NPN CLD current booster test circuit graph 1

In figure 4 we see that the DC and small signal AC voltage rejection is about 14 dB. Not as good as we would like. But the input DC voltage changes are rejected by that same amount. Figure 5 following is a transient graph of the circuit:

Figure 5
NPN CLD current booster test circuit graph 2

Here we see in Figure 5 that there is a sine wave of about 308 uV peak at the output.

Now to reduce the AC output level some more, we could add a capacitor across the zener diode. Refer to Figure 6 following:

Figure 6
NPN CLD current booster test circuit 3

Here we added a 100uF across the zener diode. Figure 7 following is a transient graph of this circuit:

Figure 7
NPN CLD current booster test circuit 3 transient graph

In Figure 7 we have added an ESR to the filter capacitor. The AC peak voltage is about 250uV. An AC sweep of the circuit produces the graph shown in Figure 8 following:

Figure 8
NPN CLD current booster test circuit 3 AC sweep graph

The AC sweep of the circuit in Figure 3 shows that the AC signal rejection starts to become effective at about 14 Hz and adds additional AC rejection.

The final test circuit is shown in Figure 9 following:

Figure 9
NPN CLD current booster test circuit 4

In Figure 9 we added resistor R5 to provide a resistance to form the resistive 'tee' filter as shown in the previous article. One side of the 'tee' is the output impedance of the CLD multiplier device itself. The other side was a 100 ohm resistor, small enough to add a DC voltage drop of only about 10V, yet large enough to give C1 some resistance to react with. A transient graph of this circuit is shown in Figure 10 following:

Figure 10
NPN CLD current booster test circuit 4 transient response

In Figure 10 we see that the output AC has a transient that lasts for about 100mS. This occurs as the maximum charging current for the capacitor C1 is about 100ma.

An AC sweep of the circuit produces the graph shown in Figure 11 following:

Figure 11
NPN CLD current booster test circuit 4 AC sweep

In Figure 11 we see that there is no appreciable difference between the response of Figure 8 and that of Figure 11. From the previous CLD paper we saw that the ratio of the two resistances that 'bridged' the filter capacitor should be somewhere between 0.3 and 0.7. But here the output impedance of the current multiplier circuit is somewhat large, making the addition of a sufficiently large value for R5 drop too much DC voltage (the 400 ohm resistor drops 40v as the CLD current multiplier output is about 100 ma.)

The conclusion is that the circuit of Figure 6 is as good as we can get, essentially, save for some other modifications.

Conclusions:

A current multiplier circuit can extend the useful output current of a CLD diode, although the output DC and AC change rejection will suffer somewhat. Some additional investigation should be performed if this circuit is used in a precision application to ensure that the noise performance is adequate.

References:

1. Central Semiconductor, SPICE models and datasheet:
http://www.centralsemi.com/spicemodels/spicecld.aspx