Measuring voltage drop across a diode in a switched circuit

I have the below circuit where the input voltage is 8V and a PWM pulse is given to the base of the Q101 transistor.

From what I have measured,

The voltage at the anode of the diodes D102 and D103 is constant like around 8V. But when I measure the voltage at the cathode of the D103 and D103, I get a PWM waveform whose peak value is 7.7V.

Voltage measurements are done using oscilloscope.

My questions :

1. How to measure the voltage drop across the diodes D102 and D103 in this senario?

2. Why am I getting a PWM voltage waveform only the cathode of the diode and not at the anode?

3. Why is there a 0.3V voltage drop across the diode? Shouldn't it be typ. 0.7V?

BOUNTY EDIT :

-The 8V is the voltage source. SBATT is connected to a load which draws a minimum current of 80mA and a maximum of 100mA.

-The temperature of the circuit , which I have measured is not 150degC. The temperature is around 85degC

-The diode is not a schottky diode.

• What frequency does the PWM signal have? Jan 8, 2020 at 7:38
• The PWM Freq = 10Hz
– user220456
Jan 8, 2020 at 8:00
• In a comment to me you claim the temperature is 60°C. Please tell which is correct and also please tell how you measured it. May 24, 2020 at 19:56
• Could you also show how you did the voltage measurements, please show scope images etc May 24, 2020 at 19:57

How to measure the voltage drop across the diodes D102 and D103 in this senario?

Like you did: with an oscilloscoop.
For example by measuring the anode of the diode on one channel and the cathode on the other channel and use the math functionality (if exist) to calculate the difference, or use the cursor to measure the difference.
Another option would be to galvanic isolate the oscilloscoop and connect to probe across the diode.

Why am I getting a PWM voltage waveform only the cathode of the diode and not at the anode?

Because the anode is connected to a voltage source which will regulate its voltage to maintain a constant 8V.

Why is there a 0.3V voltage drop across the diode? Shouldn't it be typ. 0.7V?

Yes, it should.

Only when the ambient temperature would be 150°C (which I doubt it is) the drop would be 0.3V

UPDATE with simulation as discussed in comments
If the diodes are really 1SR154-400 and noty Schottky's, the only explanation for the $$\V_F\$$ of 300mV I can think of is that these diodes are hardly conducting. When assuming SBATT is a battery which voltage is about or slightly higher than 7.7V, then that battery provides the main current through R1. The contribution of the 8V is quite low, say about 2uA, resulting in a VF of only 300 mV. See simulation below. I used 2 shunt resistors of 1 mΩ to obain the current delivered by the battery and the current though the diodes. I made assumptions for the type of Q1 and Q2.

• If I use a 12V Lead acid vehicle battery instead of the 8V Constant voltage source, will that 12V Battery still regulate and provide the constant 12V on the anode side of the diode when connected?
– user220456
Jan 8, 2020 at 8:17
• Yes. The ambient temperature while measuring the voltage drop across the diode is only 25degC.What could I be missing here as the voltage drop is around 0.3V only ?
– user220456
Jan 8, 2020 at 8:20
• @Newbie I first assume you double-checked the diodes they are indeed 1SR154-400 and not accidently Schottky diodes. The only explanation I can think of involves another assumption: SBATT is a battery which voltage is about or slightly higher than 7.7V and this source provides the main current through R1. The contribution of the 8V is quite low, say about 2uA, resulting in a VF of only 300 mV. See simulation attached in answer (where I also made assumptions for Q1 and Q2). Jan 8, 2020 at 9:25
• @Huisman Oh yes, you're right. I missed the temperature! Editing my answer. Jan 8, 2020 at 14:25
• The simulation didn't make it appelarently in the update. Ill retry postiny tomorrow Jan 8, 2020 at 18:19

How to measure the voltage drop across the diodes D102 and D103 in this senario?

You have already done this with scopemeter. This is the correct way.

Why am I getting a PWM voltage waveform only the cathode of the diode and not at the anode?

Because you are not switching the source. And since the current flows through the diode, you'll see the pwm-like signal at the cathode because of the voltage drop on it. If there were a resistor instead of a diode you'd see the pwm-like signal at resistor's right-side terminal (i.e. the one that does not connected to the voltage source).

When the Q100 is off (i.e. the PWM signal is low and Q101 is off) there will be zero current draw and thus zero voltage drop across the diode. When the Q100 is on (i.e. the PWM signal is high and Q101 is on) a current of around 5mA flows through the resistors and Q101 (assuming SBATT-side is unloaded). Thus, the diodes will show a voltage drop across them.

Why is there a 0.3V voltage drop across the diode? Shouldn't it be typ. 0.7V?

This depends on the diode itself. For silicon diodes, yes, the drop is 0.6~0.7V but under some test conditions (e.g. 5mA forward current). For schottky diodes, the drop can be as low as 0.2V. The schematic says that the diode is 1SR154-400. The current flowing through the diodes is around 5mA, so 2.5mA per each (assuming they are nearly identical). If you check VF-IF characteristics from the datasheet, VF is around 0.3V for IF = 2.5mA. Assuming the model number of the diode given in the schematic is correct, the only possibility of low voltage drop could be a damaged diode. Or else the diode is a schottky diode. By the way, parallelling two discrete diodes is not a good idea.

• Thank you for the answer. But please clarify when you said, even if I connect a resistor in place of a diode, the resistor right side only will have PWM voltage level and not the left side(which is connected to the voltage source). Can you explain a little bit more on this?
– user220456
Jan 8, 2020 at 8:05
• VF is around 0.3V at an ambient temperature of 150°C!! Jan 8, 2020 at 8:11
• Current flowing through the diode is dropping that voltage. Same if you connect a resistor: when you let current flow through it, a voltage drop is what you measure. Since you have a Constant Voltage source (8V) connected to one terminal, that voltage source holds the voltage constant at that terminal. At the other terminal you see the voltage dropped. Jan 8, 2020 at 8:12
1. How to measure the voltage drop across the diodes D102 and D103 in this scenario?

On a DSO with 2 probes use CALC Ch A-B

1. Why am I getting a PWM voltage waveform only the cathode of the diode and not at the anode?

Anode is connected to a voltage source which in theory is a 0 Ohm to AC but Cathode is high impedance.

1. Why is there a 0.3V voltage drop across the diode? Shouldn't it be typ. 0.7V?

When there is no apparent DC load and diode Vf=0.3 V , this indicates there is < 100uA leakage current which is due to Rce across the collector emitter called the Early Effect leakage resistance.

• There is a load of 80mA to 100mA at the SBATT terminal. The voltage - 8V only is providing this load current
– user220456
May 23, 2020 at 9:23
• It is std Silicon and you can expect Vf to drop with heat (thermometer effect) But even at 125'C max it should be > 0.6V . @ 40mA see datasheet rohm.com/products/diodes/rectifier-diodes/standard/… May 23, 2020 at 19:59

There should be a drop of about 0.7V across those diodes, when there is nonnegligible current through them. When Q100 is on and you have a load connected, it should be down at about 7.3V.

In a perfect world, there would be no drop across the diodes when Q100 is off. However, a little bit of extrapolation on the curves indicates that a current of 1-10uA could result in about a 0.3V drop at 25C. This is in the neighborhood of leakage currents and it wouldn't be surprising to get this result.

So as you switch off and on, you would see the diode cathodes switch from 7.3V to 7.7V (you did say 7.7V was the peak, not the valley). At the same time, the output would switch from about 0V to 7.1V (due to the 0.2V saturation of the PNP).

Does this match what you're seeing on your scope?

• Good point +1 .I have seen this on a DVM measuring 400mV across some fast diodes but the overall duty cycle was less than 50% . Jan 18, 2020 at 4:33
• There is a load of 80mA to 100mA at the SBATT terminal. The voltage - 8V only is providing this load current
– user220456
May 23, 2020 at 9:24