Clifton Laboratories 7236 Clifton Road  Clifton VA 20124 tel: (703) 830 0368 fax: (703) 830 0711

E-mail: Jack.Smith@cliftonlaboratories.com

This page presents turn-on and turn-off time measurements for five power diodes:

• 1N4005
• 1N4007
• 1N4001
• HEP108
• 11SQ05

I took these measurements to answer a debate on the PIC mailing list concerning the turn-on time of typical silicon power diodes. In particular, whether a silicon power diode, such as the common 1N400x series, has an appreciable turn-on delay such that it renders it unsatisfactory to clamp the inductive kickback of an inductor, such as a relay coil, when switched with a MOSFET.

As a preliminary matter, it's well known that standard silicon power diodes, such as the 1N400x series have a significant turn-off time, commonly called the "reverse recovery time."

The reverse recovery time involves a diode switching from forward biased (conducting) to reverse biased (non-conducting). The diode's PN junction, when conducting, has excess minority carriers and it requires a finite time for these excess minority carriers to be neutralized when the PN junction switches from forward to reverse bias. For more detail, see http://www.microsemi.com/micnotes/302.pdf. Standard silicon power diode, intended for use in 50/60 Hz power systems, exhibit a reverse recovery time of several microseconds, although higher performance devices are available.

Less common considered is the forward recovery time, i.e., the time it takes the diode to conduct when switched from reverse to forward bias. This mode is involved in the typical inductive clamp circuit, as can be seen from the circuit fragment below. When the relay coil is energized (the 2N7000 MOSFET is biased into saturation), diode D8 is reversed biased. When the 2N7000 MOSFET is turned off, the relay's magnetic field collapses and induces a reverse voltage in the windings, which forward biases D8. (Lenz's law says the induced voltage is reversed.)

If D8 has a slow forward recovery time, it is possible for the voltage induced by the collapsing field to rise to levels that might damage the switching MOSFET before D8 begins conducting. (This is a simplification, as the MOSFET will breakdown at some voltage level and will thus limit the voltage. The question is whether the energy deposited into the MOSFET in this event is sufficient to damage the device. We'll ignore this aspect of the analysis.)

The 1N4005 is a member of the venerable 1N400x family of silicon rectifiers. I don't know when they were first introduced, but I recall purchasing some in early 1970's so they have been around for 35 years plus. Of course, today's 1N400x diode is not necessarily the same as one fabricated 35 years ago. I have no idea when the 1N4005 I tested was manufactured, as it's been in my junkbox for an unknown time. (My 1971 GE Transistor/Diode handbook lists the 1N400x series, so the original introduction date must have been in the mid to late 1960's.)

Fairchild's data sheet for the 1N4005 (the datasheet is for all 1N400x series) can be viewed at http://www.fairchildsemi.com/ds/1N/1N4005.pdf. It notes the 1N4005 has a reverse voltage breakdown of 500 volts, providing no data on either forward or reverse recovery time. Like all other members of the 1N400x family, the 1N4005 is rated at 1 A forward current.

The horizontal sweep time is 1 μs/div. The vertical spikes show the applied pulse transition times. Voltage above the center graticule line are positive, indicating forward bias and those below are negative, where the diode is reverse biased.

Note how fast the transition from reverse bias to essentially steady state forward conduction is. The forward bias transition is way too short to see with the 1 μs/div sweep speed. There's a slight rise in forward voltage for 200 ns or so after the diode transitions from reverse to forward bias, but it's only a tenth of a volt or so, negligible in the application being considered.

The reverse recover time, in contrast, is remarkable. Note how the 1N4005 continues in forward conduction state essentially unchanged for 400 to 500 ns after the driving waveform switches from positive to negative, with only a slight drop in voltage. After 500 ns or so, the diode begins to cease conduction, although the total time to stabilize in reverse mode approaches 3 μs.  Of course, the reverse recovery time is not of significance in an inductive clamp application.
 

The 1N4007 is the 1000 V reverse voltage member of the 1N400x family, with the same 1 A forward current specification.

As the illustration below shows, it's not much different than the 1N4005. The forward recovery time is very short, on the order of a few nanoseconds, with a similar small transition after switching to forward conduction.

The reverse recovery time is similar to the 1N4005.

It's commonly understood that the 1N4007 has an special junction structure resembling a PIN diode and it has, of course, been widely used as a "poor man's PIN diode" in RF power switching. Elecraft, for example, uses 1N4007 diodes for transmit/receive switching in both its 10 watt and 100 watt option K2 transceiver.

Used as a quasi-PIN diode for RF power switching, the 1N4007 takes advantage of the lengthy reverse recovery period, which prevents it from rectifying the high frequency signal it is switching. (This is because the reverse voltage half-cycle period of, say, a 3.5 MHz signal is much shorter (140 ns) than the reverse recovery time. Hence, if forward biased with DC, any reverse bias from RF negative half-cycles don't last long enough to neutralize the 1N4007's excess minority carriers. Hence, the 1N4007 stays forward biased and presents a low impedance to the full RF cycle. There's a minimum frequency for this effect which we see from the trace below. With a bias current in the 60 mA range from the 8012B pulse generator, we see the following voltage waveform.
 

The figure shows during the forward bias period, the current is 67.5 mA. Then, when the 8022B pulse generator reverses polarity and reverse biases the diode, we see the current approximately doubles during the recombination time. This can be considered as the sum of the recombination current in the PN junction (68 mA) plus the negative current from the pulse generator (68 mA) or a total of 136 mA. As the excess minority carriers are neutralized, the diode assumes its normal reverse bias state of being essentially non-conducting, with negligible current flow.

Note that since the probe measures current, the forward turn on voltage elevation appears as a reduced current.

Spehro Pefhany has provided a link to the forward recovery time graph presented below for the 1N400x series diodes. Unfortunately, my 68 mA forward test current is just off the horizontal scale, so we have to extrapolate a bit. Also, the magnitude of the Vf elevation during the forward recovery time is not provided.

Nonetheless, the forward recovery time graph is in general agreement with the data I measured.

The transition A' to A takes the pulse generator goes from negative to positive, and thus the diode from reverse bias to forward bias. The 'scope sweep shows the transition to be quite fast, as A'-A appears as an essentially vertical line. With 500 ns/div, we expect to see a transition time that is over 50 ns to show as a sloping line, so we can say that the 1N4007's turn-on time, or forward recovery time is < 50 ns. At most, we see a small elevated forward voltage effect for a couple hundred ns after the diode becomes forward conducting. The diode's forward voltage starts around 1.2 volts, dropping to the steady state level of 0.8 volts within 250 ns or so.

At time B, the pulse generator output switches from positive to negative. When negative, the 1N4007 is reverse biased and should carry no current. Accordingly, we should see on the oscilloscope the full negative pulse voltage, around -4 volts at time B. However, the 1N4007 continues in forward conduction at time B, and indeed even by time C, the diode voltage is still > 0. In other words, during the time from B to C (approximately 500 ns) the 1N4007 is still in forward conduction mode, despite the fact that the diode is reverse biased to -4 volts by the pulse generator. Rather, the diode stays in forward conduction during the time B-C as it takes time for the excess minority carriers in the 1N4007's PN junction to be neutralized by the reverse bias.

In fact, it takes until time D for the last excess minority carriers to be neutralized and the 1N4007 to become fully reverse biased with essentially no current flow. This is a full 3 μs between the time the diode is reverse biased and the time that current flow actually ceases.

The 1N4001 is the lowest voltage rated of the 1N400x family at 50 volts. It's reverse recovery switching time behavior is similar to other members of the family, as evidenced in the curve below.

However, the 1N4001's forward switching shows considerably less of the increased forward voltage effect seen in the 1N4005 and 1N4007 devices.
 

• The turn-on time is about 4 ns, which represents the 2246's bandwidth (100 MHz) and the rise time of the 8012B pulse generator (less than 5 ns). The 1N4001's turn-on time is thus less than the test equipment used in my test setup.
• The excess forward voltage, Vf, is essentially zero. The overshoot and ringing shown in the 20 ns/div sweep is likely an artifact of my test setup and cable ringing.

The HER 108 is a member of the HER 10x family, similar to the 1N400x group, with the part number's last digit increasing with increasing voltage. The HER 10x diodes are all rated at 1.0 A forward current, and the HEr 108 is rated at 1000 V reverse voltage. For more detail, see Rectron's data sheet for the family at http://www.rectron.com/data_sheets/her101-108.pdf  .

However, the HER 10x family is designed to address the reverse recovery problem seen in the 1N400x and other similar run-of-the-mill silicon power rectifiers, with a maximum reverse recovery time of 50 ns (for the HER 101-105 devices, through 400 volts) and 75 ns for the HER 106 - 108 parts (600 - 1000 volts).  No forward recovery time is defined in Rectron's data sheet.

The illustration below confirms the major improvement in reverse recovery time compared with the 1N400x diodes.

The forward recovery time is still short, on the order of a few nanoseconds, but clear signs of different behavior are seen. The initial conduction voltage is around 2 volts and it does not drop to the steady state value of 1 volt for about 500 ns. Both the voltage elevation and response time in this regard are worse than the 1N400x devices. However, for diode clamping, the HER 108 seems adequate. Of course, there's no real reason to pay for the HER 108's improved reverse recovery time in this application.
 

11DQ05

The 11DQ05 is a 1.1 A, 50 volt Schottky diode and more details may be found at http://www.vishay.com/docs/93206/9320611d.pdf.

We expect a Schottky diode to have fast forward and reverse recovery times, and the 11DQ05 does not disappoint.

To better see the 11DQ05's performance, I've increased the sweep speed to 0.5 μs/div in the image below. As the illustration shows, both the forward and reverse recovery times are not easily measurable with this sweep speed, and are on the order of a few nanoseconds.

Also, the Schottky's lower forward bias voltage drop shows clearly, with about 0.4 volts across the diode when forward biased.

There's a clear difference in reverse recovery time amongst the diodes examined, with the old 1N400x series having by far the worst reverse recovery time. The HER 108 improved silicon diode provides a much better reverse recovery time, and the 11DQ05 Schottky is even better.

However, for a diode to be used as an inductive snubber, the evidence is not as clear. All of the diodes transition from reverse to forward bias with great rapidity. At worse, the HER 108 has a period of higher than normal forward voltage, but the elevated voltage is not that great when considered in the context of an inductive snubber.