Choosing & Using Pulsed Constant-Current Sources, and Blue Diode Considerations

Introduction

The common lab pulse generator is designed to supply pulses of a certain voltage to a resistive load. However, there are many applications where the engineer requires a pulse of a certain current which must remain constant in amplitude, independent of the load voltage or resistance. This is a common requirement in applications as diverse as the testing of superconductors, airbag squibs and inflators, fuses, explosives, laser diodes and a wide variety of semiconductor devices. These applications often require fairly large currents of a few amps or more. Avtech has several model families ideally suited to these pulsed constant-current applications (see Table 1 below and pages 66 to 73).

Table 1

Model Iout, max (A) Compliance Voltage Maximum Current Variation Pulse Width Rise Time Fall Time Max. Duty Cycle (%) Max. PRF (kHz)
AV-156A-B

5

0 to 15V (25V opt.)

< 2%

10us-10ms

4us

4us

20

10

AV-156G-B

10

0 to 15V (25V opt.)

< 2%

10us-10ms

4us

4us

10

10

AV-151F-B

±2.5

0 to ±5V

< 1%

func. gen.

10us

10us

100

20

AV-106A-B

30

0 to 30V

< 10%

0.5-50us

50ns

50ns

0.25

1

AV-106B-B

100

0 to 100V

< 10%

2-200us

1.0us

1.0us

0.1

0.1

AV-106C-B

15

0 to 20V

< 10%

1us-1ms

50ns

50ns

1

1

AV-106D-B

5

0 to 5V

< 10%

1us-1ms

0.5us

0.5us

50

1

AV-107B-B

2

0 to 60V

< 5%

2200 ns

10ns

10ns

0.4

20

AV-107C-B

10

0 to 60V

< 5%

50ns-1us

20ns

20ns

0.5

5

AV-107D-B

20

0 to 60V

< 5%

0.1-5.0us

30ns

30ns

0.25

0.5

AV-107E-B

2.5

0 to 60V

< 5%

0.2-200us

30ns

30ns

20

1

AV-108E-1A-B

50

0 to 20V

< 5%

20us-1ms

10us

10us

4

1

AV-108F-1A-B

50

0 to 20V

< 5%

20us-10ms

10us

10us

40

1

AV-108E-2A-B

100

0 to 50V

< 5%

20us-1ms

10us

10us

0.8

1

AV-108F-2A-B

100

0 to 50V

< 5%

20us-1ms

10us

10us

8

1

AV-108E-3A-B

200

0 to 20V

< 5%

20us-1ms

10us

10us

1

1

AV-108F-3A-B

200

0 to 20V

< 5%

20us-1ms

10us

10us

10

1

Compliance Voltage and Maximum Amplitude Variation

Aside from the obvious current pulser specifications such as maximum current amplitude, the rise time, and the pulse width range, there are two other key parameters: compliance voltage and the maximum amplitude variation. The compliance voltage, VC, is simply the ranges of load voltages that the pulsed constant-current source will work properly with. For instance, the Avtech AV-108F-1A-B pulsed constant-current generator is specified as having a maximum amplitude of 50 Amps, and a compliance voltage of 20 V. The AV-108F-1A-B will function properly only if the load voltage remains below 20 V. As an example, if the amplitude of the AV-108F-1A-B was set at 40 A, the largest resistive load that could be used would be 0.5Ω , since 40A x 0.5Ω = 20 V. (The smallest usable resistive load is 0Ω - a current source is not harmed by a short circuit, unlike some voltage pulsers.)

The second key parameter is the maximum variation of the current amplitude with a change in the load voltage. An ideal current pulser has no current amplitude variation when the load voltage changes. However, most current pulsers will in reality display a slight change in the current. This change is usually specified as a percent change in the current when the load voltage is changed from the zero Volts to the compliance voltage VC. This is the worst case variation. As an example, if the AV-108F-1A-B were set at 40 Amps, into a short circuit (zero Ohms), and the load resistance then rises to 0.5Ω (perhaps due to thermal effects, or the opening of a switch) where the load voltage is equal to VC, then the current is specified to changed by no more the 5% - it will lie in the range of 38 to 40 A.

Capacitive Loads

If the load has a large shunt capacitance, the time to charge up the capacitor voltage will often be limited by the laws of physics rather than the pulse generator rise time. The defining equation for a capacitor is I = C dV/dt. For instance, consider the AV-107E-B pulse generator, which will supply pulses of up to 2.5 A with a specified rise time of less than 30 ns (see upper waveform, Figure 1). However, if this pulsed constant-current generator is set at a low amplitude of 20 mA and is used to drive a 1kΩ load, a voltage risetime of about 1 us is observed (see bottom waveform, Figure 1). The voltage rise time is limited by the parasitic output (and load) capacitance (which can exceed several hundred picofarads) through the relationship I = C dV/dt. If the load were a laser diode, this parasitic capacitance would delay the time for the diode voltage to reach the lasing threshold voltage.

The fall time of a current pulser may or may not be affected by the presence of capacitance, depending on the particular model that is used. For the example discussed above (see bottom waveform, Figure 1) the fall time is still very short, because the pulser output is shorted to ground when the output isn't supplying a current pulse. This discharges any capacitance very quickly. Other models do not short the output to ground, so the fall time is controlled by the I = C dV/dt relationship, just like the rise time.

Figure 1

Waveforms illustrating the effect of inductive and capacitive parasitic elements on the output of Avtech current pulsers

Inductive Loads

While capacitive loads may degrade low current pulses, the true mortal enemy of current pulsers is load inductance. Extreme care has to be used when using current pulsers, and especially higher-current, higher-speed ones, with any sort of inductive load or cabling. Even a small inductance can generate a significant "inductive kick", which is a voltage spike predicted by Lenz's Law: V = L dI/dt. As an example, consider the situation where an Avtech AV-107E-B is used to drive a resistive load of 10Ω , and the load is connected 4" away from the generator, using 20 AWG wire on the signal line and the ground return, for a total of 8" of wire. The AV-107E-B will provide 2.5 Amps of current with a rise time of 30 ns, and the wire will have an inductance of approximately 200 nH. Lenz's Law then predicts an inductive voltage spike of 15V! (See middle waveform, Figure 1). Even if the total lead length is reduced to 1 inch, the inductive spike will still be over 1 V!! (1 inch of #24 wire equals 20 nh)!! There are two complementary approaches to combat this problem. This first approach is to place the load extremely close to the pulse generators, and avoiding the use of cabling or connectors. For the ultimate in performance and convenience, socket-mounting of the load is available on many models. (The specially-designed low inductance socket allows a diode load to be mounted directly on the pulse generator output module.) The second approach, discussed below, is to use a low impedance transmission line.

Low Impedance Transmission Lines

Transmission lines are characterized by a parameter Z0, the "characteristic impedance" of the transmission line. When a transmission line is connected to a load that is equal to its characteristic impedance (RL = Z0), then the transmission line acts as a perfect cable: the pulse generator does not "see" the parasitic capacitance or inductance in the line. This technique can be used for lengths of up to a few feet. The primary difficulty lies in obtaining a transmission line of the correct impedance - virtually all commercially available cables have impedances of 50Ω or higher. Laser diodes and other high-current loads are more likely to have resistances of only a few Ohms. Fortunately, Avtech produces a line of low-Z0 transmission lines (the AV-LZ series, see page 77). These are available with Z0 values of 1, 2, 3, 6, and 12Ω . The AV-LZ lines are available with a diode socket option, to allow convenient mounting of a diode load at the end of the transmission line.

Current Pulsers Are Lossy

Current pulsers are inherently lossy - that is, they dissipate much more waste heat than voltage pulsers. For this reason, current pulsers are often limited to low duty cycle operation, and/or use water and fan cooling. (Duty cycle is the fraction of the time that the pulser output is high - i.e. 100% x pulse width / period). The worst-case instantaneous power dissipation in current pulsers occurs when the load is a short-circuit, and is approximately equal to the compliance voltage times the current amplitude. For instance, the Avtech AV-108F-3A-B will supply 200A pulses with a compliance voltage of 20V, leading to a worst-case instantaneous power dissipation of 4 kW! (The maximum allowed average power dissipation in the AV-108F-3A-B is 400W, which results in duty cycle limitations.)

Current Monitors

Measuring and observing current pulses can be awkward, since oscilloscopes are designed to measure voltages. Several approaches are available. For example, probes and current transformers are available from a number of sources, such as Tektronix, Pearson or American Laser. The probes and transformers can provide excellent results but note that the current-carrying conductor must be fed through the donut-shaped transformers or probes, so they can not be used with transmission lines, since the main conductor is shielded. A second approach employs low-inductance current-sensing resistors (such as those available from Isotek and Caddock) which are placed in series with the load, and the voltage across the resistor will therefore be proportional to the current. The resistance must be kept low to avoid large voltage drops and power dissipation. The third method relates to the fact that many Avtech current pulsers are available with convenient built-in current monitors. The current monitor output supplies a voltage pulse that is proportional to the main output current pulse (with equal pulse width). In addition, many units with the -B GPIB feature provide an LCD readout of the current.

Figure 2 shows four waveforms for an Avtech current pulser. The top waveform shows the voltage across a 5Ω load when a constant pulse of 400 mA is applied. The second waveform shows the load voltage when a 0.5Ω load is used instead, with the same current amplitude. Naturally, the load voltage is ten times smaller, since the resistance is ten times smaller than before. The third and fourth waveforms show the output of the current monitor for these two cases. These two waveforms are identical, since the current amplitude has not changed, despite the different load resistances and voltages. (In this example, the current monitor output provides Vmonitor = IOUT/5. Since IOUT = 0.4A in both cases, Vmonitor = 80 mV, as shown in the photo.)

Figure 2

Waveforms illustrating the operation of the current monitor feature available in some Avtech pulsed constant current generators

Using a Voltage Pulser as a Current Pulser

It should be noted that if the user does not require a current of more than one or two amps and can tolerate some current amplitude variation with load voltage, an Avtech 50 or 100 Volt pulse generator can be used to approximate a pulsed constant current-source source by adding a resistance in series with a laser diode load. Figure 3 illustrates this technique. The Avtech AV-1010-B pulse generator is designed to deliver 100V pulses into a 50Ω load. If the 50Ω load is replaced with a 49Ω resistor (RSERIES) in series with a 1Ω laser diode (RD), the AV-1010-B could be used as a 2 A current source (since 100V / RSERIES+RD = 2A). This setup has the advantage that a 50Ω transmission line can be used, since the transmission line will be terminated in 50Ω . Relative to a true current source, this approach is far less subject to inductive kicks (i.e. LENZ’S LAW).

When using the AV-1010-B for much lower load currents (e.g. 10 mA) RSERIES may be increased significantly (e.g. to 10kΩ ), but then a shunt resistor (RSHUNT) must be added to the circuit as shown in Figure 4. This resistor is chosen so that the parallel combination of it and RSERIES+RD is equal to 50Ω , to properly terminate the transmission line. (If the transmission line is not terminated properly, ringing and overshoot will occur on the pulse. See Applications Note 1A, Fig. 11). When using the AV-1010-B in such applications, the same effect can be achieved simply by setting the source impedance switch to the 50Ω position (i.e. no shunt resistor is necessary).

Figure 3

Diagram illustrating the use of an Avtech AV-1010 with a series resistance to provide high output impedance, and a shunt resistor to provide proper transmission line termination

Figure 4

Diagram illustrating the use of an Avtech AV-1010 with a series resistance to provide high output impedance and proper transmission line termination

Figure 5

Diagram illustrating the use of an Avtech AV-1010 with a pulse transformer to achieve 4 Amp pulses

Figure 6

Diagram illustrating the use of an Avtech AV-1010 with a pulse transformer to achieve 8 Amp pulses

The AV-1010-B has rise and fall times of 10 ns, repetition rates from 10 Hz to 1 MHz, pulse widths from 100 ns to 1 ms and high duty cycle ratings. These wide ranges make the AV-1010-B a more versatile instrument than many of the other pulsed constant-current generators and laser diode drivers discussed above.

Furthermore, by using accessory matching transformers, pulses of up to 4A (see Figure 5) or 8A (see Figure 6) can be obtained provided the maximum pulse width is less than 10 us. The increased current comes at the expense of decreased output voltage (50V maximum for the 4A setup, and 25V for 8A). The output voltage must be kept above the laser diode threshold voltage for the diode to lase. See pages 52 to 65 for Avtech’s full line of pulsed voltage laser diode drivers.

Pulse drivers for blue laser diode research

The requirements for blue laser diode pulse mode testing are demanding because state-of-the-art research prototypes exhibit high threshold currents and high threshold voltages. Typically, the on-voltages for GaN lasers are 10 V to 30 V, or even higher depending on the degree of contact sophistication. (Diodes constructed from II-VI systems like ZnSe tend to have lower forward voltage drops, typically 4 V to 12 V). The threshold current depends largely on the diode size and power and can vary from several tens of milliamps to several amps.

Avtech has several families of pulsed constant-current generators that will provide these large currents and large voltages simultaneously (see Table 1). For example, each of the four models in the AV-107 series has a compliance voltage rating of 60 V, which is more than adequate for most blue laser diode applications. These models feature moderately fast rise and fall times of 10 to 30 ns and peak current rating of 2 to 20 Amps. The AV-106 series units offer similar current and voltage ratings but with higher rise times and wider pulse widths. For still wider pulses and lower rise times (and higher duty cycle ratings), the AV-156 series should be considered. All of the instruments in Table 1 have the feature that they are true current sources. That is, the current output is nearly independent of the load voltage or impedance. For blue diodes with lower on-voltages (and resistance), a true current source may not be necessary and one can then consider pulsed voltage drivers such as the AVO-2, AVO-5 and AVO-6 series (see pages 53 to 57) which can offer rise times as low as 1 ns and output currents in the range of 1 to 18 Amps. If rise times of 10 ns are acceptable, then the general-purpose Model AV-1010-B should be considered for applications requiring currents as high as 8 Amps. For much higher currents (to 14 Amps), the AVR-4, AVR-5 and AVR-7 series (see pages 40 to 43) should be considered.

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