Troubleshooting Switched Mode Power Supplies With A Digital Oscilloscope
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(Slides from Live webinar on September 25, 2014, presented by Mike Schnecker. Watch the webinar On-Demand here: http://goo.gl/LkjUUg) Attendees Will Learn: An overview of switched mode power supplies Common measurements (ie, what to measure and why) Circuit loading and probing considerations How instrument specifications impact measurement accuracy Switched mode power supplies have become ubiquitous in electronics as they provide precise voltages including high power with very high efficiency. The efficiency of these power supplies requires low loss power transistors and the design requires measurement of highly dynamic voltages. Voltage levels can vary from millivolts to hundreds of volts in some applications. In this webinar, the proper use of a digital oscilloscope to accurately measure these voltages will be discussed along with key aspects of instrument performance such as noise and overdrive recovery that affect the accuracy of the measurement.
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Troubleshooting Switched Mode Power Supplies With A Digital Oscilloscope
- 1. Switched Mode Power Supply Measurements
- 2. Agenda In this workshop we’ll be learning ı SMPS background and basics ı Measurement setup ı Oscilloscope measurements FAST: Advanced Triggering Averaging, filtering, grids Probing and bandwidth Current measurements and deskew ı Measurement Example: startup waveforms output voltage ripple transient behavior switch node voltage and current 9/10/2014 2
- 3. Switched mode power supply basics l Basic DC-DC converter l Switches A and B alternately charge and discharge inductor through load l Switches are realized using power MOSFET, IGBT and diodes SMPS | 3 Vs(t)
- 4. Voltage regulation in SMPS Vs(t) Vg DTs (1-D)Ts l Average voltage at the load is controlled by the duty cycle D l Waveform assumes an ideal switch SMPS | 4 0 Vs = DVg
- 5. Understanding Power Flow and Topology 10.09.2014 5
- 6. Inductor Current Waveform SMPS | 4
- 7. Measurement Setup Oscilloscope: 500 MHz or more 9/10/2014 FAST: Advanced Triggering 7 SMPS Passive voltage probes Single ended and differential active probes De-skew fixture Current probes Programmable power supply variable current and voltage remote sensing
- 8. Maximizing measurement accuracy ı Large dynamic range required for accurately measuring switching loss On state is tens to hundreds (even thousands) of volts Off state is often only several mV to a few volts Typical A/D converters provide only 6 to 8 effective bits (50 dB S/N) This is equivalent to 20 mV out of 5 V ı Maximizing signal to noise Waveform averaging High resolution and filtering filtering (trade off sample rate and bandwidth for S/N) Multiple grids ı Probing and bandwidth SMPS contain high slew rate signals and high frequency content Probing is critical for accurate measurements – bandwidth and connection Oscilloscope bandwidth and sample rate must be high enough to measure fast edges and high frequency interference
- 9. Waveform averaging ı Increases resolution by averaging samples Effective in reducing thermal (random) noise Will distort time varying waveforms Can also reduce displayed rise time Can not reduce deterministic noise sources such as interleaving artifacts
- 10. High Resolution Mode or Digital Filter ı Combine consecutive samples from A/D converter ı Preserves real time sampling – no smearing of dynamic signals ı Reduces bandwidth based on decimated sampling rate ı Should be combined with filtering to reduce interpolation error Combine samples for each point
- 13. Resolution is Reduced by Half… Half scale waveform Full scale waveform
- 14. Passive Probes – Ground Lead Length Long ground lead Short ground lead
- 15. Active Probes
- 16. Slew Rate and Vertical Resolution ı Both vertical and horizontal resolution are critical High slew rates Measuring short, high amplitude peaks that could damage active components ı 4.4 V/ns = 880 mV per sample @ 5 Gs/s ı 4.4 V/ns = 4.4 V sample @ 1 Gs/s ı Compare to digitizer range 39 mV @ 8 bits 2.4 mV @ 12 bits ı Measurement is limited by the sampling rate
- 17. Slew Rate and Vertical Resolution ı Use high bandwidth probe Shortest lead lengths Active probes if possible ı Maximize sampling rate and bandwidth Sampling rate 5 to 10 times the scope bandwidth Oscilloscope rise time 10x faster than switch time ı Use averaging whenever possible High resolution mode reduces rise time, bandwidth and sampling rate Averaging preserves sampling rate and rise time
- 18. FAST: Advanced Triggering Measuring Current ı Clamp-on current probes Both DC and AC current measurement Must be “de-magnetized” Requires a loop in the circuit Limited bandwidth ı Shunt resistor Measure the voltage drop across a small resistor – usually 0.1 ohm Resistor must have stable value over temperature and current Highest bandwidth ı Indirect method using near field probe Only AC current proportional to -d(i(t))/dt Limited sensitivity Measurement on very small geometry and without disturbing the circuit 9/10/2014 18
- 19. Using a Near Field Probe to Measure Current Current flow H field ı Probe voltage proportional to the derivative of the current ı Small form factor probes can reach tight spots ı Integrate signal to measure current ▪ Integral reduces noise on small signal Vo
- 20. Positive voltage vs current pulse skew Deskewing with reference voltage and current pulses essential for accurate power measurements RT-ZF20 - Power Deskew Fixture Probe De-skew ı Skew between voltage and current probe leads to wrong power measurement results Feb. 2013 20 Power measurement too low Negative voltage vs current pulse skew Power measurement too high Positive voltage vs current pulse skew Power measurement too low Deskewed, accurate measurement
- 21. RT-ZF20 – How to deskew 1. Connect RT-ZF20 to USB 2. Connect current probe and voltage probe Voltage pulse Current pulse Deskew Different propagation delay between current and voltage pulse Current and voltage pulse aligned RT-ZF20 - Power Deskew Fixture to RT-ZF20 3. Overlay current and voltage pulse Trigger condition rising + falling edge Adjust vertical scale to same pulse height 4. Adjust „Deskew“ parameter of scope for current probe Feb. 2013 21
- 23. FAST: Advanced Triggering Startup Waveforms 9/10/2014 23 No load 5 W load 20 W load
- 24. Output Voltage Ripple and Spectrum
- 25. Measure output voltage ı Measured using passive probe with long ground lead 9/10/2014 FAST: Advanced Triggering 25
- 26. Measure output voltage ı Measured using passive probe with short ground lead 9/10/2014 FAST: Advanced Triggering 26
- 27. Measure output voltage ı Measured using passive probe with an active probe 9/10/2014 FAST: Advanced Triggering 27
- 28. Measure output voltage spectrum ı Spectrum measured out to 30 MHz ı Spurs look very similar in both cases 9/10/2014 FAST: Advanced Triggering 28 20 Ω load 5 Ω load
- 29. Measure output voltage spectrum ı Spectrum measurement up to 500 MHz ı Increased noise between 100 and 300 MHz with 5 ohm load 9/10/2014 FAST: Advanced Triggering 29 20 Ω load 5 Ω load
- 31. Measure The Voltage Transient Response ıOutput voltage during load transient 9/10/2014 FAST: Advanced Triggering 31 No load 20 Ω load 5 Ω load 4 Ω load
- 32. Measure The Voltage Transient Response ı Examining voltage after filtering ı Stability is determined by analyzing overshoot and any ringing ı Can be measured in-circuit 9/10/2014 FAST: Advanced Triggering 32
- 33. Measuring Switch Node Voltage and Inductor Current
- 34. Measure Switch Node Voltage and Current ı 20 ohm load ı Current measured using near field probe ı Math waveform computes integral of near field probe voltage ı Averaging and high resolution mode applied to signals 9/10/2014 FAST: Advanced Triggering 34
- 35. Measure Switch Node Voltage and Current ı 5 ohm load ı Current measured using near field probe ı Math waveform computes integral of near field probe voltage ı Averaging and high resolution mode applied to signals ı Slope of voltage increased compared to 20 ohm case and inductor current is non-linear 9/10/2014 FAST: Advanced Triggering 35
- 36. FAST: Advanced Triggering Conclusion ı Increasing the load to 5 Ω results in reduced voltage (by approximately 200 mV) and increased voltage ripple Increased spectral power above 100 MHz 3% ripple voltage ı Examining the switching node revealed that the inductor appears to be the root cause Non-linear IL with 5 Ω load Decreased rise time of Vsw with increased load Higher slope on Vsw at higher load ı The problem is traced to an undersized inductor 9/10/2014 36
Editor's Notes
- The circuit shown here is a basic DC-DC converter also known as a buck converter because the output voltage Vs is lower than the input voltage Vg. The two switches operate synchronously so that switch a is open when switch b is closed and vice-versa. The inductor and capacitor form a low pass filter that removes all of the switching frequencies leaving only the DC component at the load resistor R. The inductor can also be viewed as an energy storage element charging up when switch a is closed and discharging through the load when switch a is open. The switches are realized using semiconductor devices such as FET’s, IBGT’s or diodes.
- Voltage regulation is determined by the duty cycle of the switches The trace shown here is that of the voltage at the load without the inductor or capacitor present. The voltage switches between Vg and zero with the average being determined by the duty cycle D. This waveform is assumes that the switches are ideal in that they switch instantly and there is no loss in the “on” state.
- This screen shot from an RTM oscilloscope shows a few key measurements on a buck converter. The blue shaded areas correspond with the switch grounding the inductor while the tan areas correspond with the switch applying the source voltage to the inductor. The switch voltage alternates between Vs and 0 V while the inductor current varies linearly as the inductor charges and discharges. The ouput voltage ripple folliws the switching and is minimized via the filtering of the LC network. Note the scale of the output voltage (on channel 4 in this case) is 20 mV/div
- The overall inductor current is then a triangle wave with slope alternateing between (Vg-V)/L and –V/L.
- The large switching voltages require a high bandwidth oscilloscope typically in the range of 1 GHz to measure. Oscilloscopes in this range typically use an 8 bit A/D converter with 6 to 7 effective bits of resolution. Effective bits is a way of expressing the effect of noise and distortion on the dynamic range of the instrument. For example, a 43 dB signal to noise ratio corresponding to a 6.8 effective bit A/D results in approximately 70 mV of RMS noise on a 10 V full scale signal. This level of uncertainty is generally too high to measure the drain to source voltage which is usually in this range. There are three methods employed to improve the resolution of oscilloscopes for this measurement. Waveform averaging can be used to reduce trace noise or high resolution mode decimation can be used to trade off bandwidth for signal to noise. In some cases, the instrument can be overdriven to increase the accuracy of measurement of small signal levels. Each of these methods has drawbacks which will be discussed.
- Waveform averaging can be used to decrease the trace noise significantly. This type of averaging works on consecutive waveform updates. Random variations in the waveform from update to update are reduced by finding the mean at each time sample across the waveform. This type of averaging is dependent on the waveform being perfectly repetitive. Any variation in the waveform over time will result in distortion. This is the waveform of the switching voltage (drain to source voltage) of a power FET. Because the duty cycle is being continuously adjusted to regulate the voltage, only the pulse in the center at the trigger point is not distorted. The other two pulses are severely distorted by the averaging. Trigger jitter in the oscilloscope can also cause the signal to move at the trigger point which can induce distortions which limit the amount of averaging that can be employed.
- Another method that can be employed by oversampling A/D converters is referred to as enhanced resolution or high resolution mode. This mode combines consecutive samples from the A/D converter into one sample. For example, 100 consecutive samples the 10 Gs/s A/D in the R&S RTO oscilloscope can be combined (averaged) to provide an increased signal to noise by lowering the effective sampling rate to 100 Ms/s. The lower sampling rate is accompanied by a low pass filter to keep the signal bandwidth below 50 MHz. The main advantage of high resolution decimation mode is that the dynamic signal behavior is preserved since no averaging is done between acquisitions. The upper figure shows how the high resolution mode combines samples and reduces the sampling rate. The lower image shows the drain to source voltage of the switched mode power supply. The waveform shape is preserved even for this highly dynamic signal. History mode can be used to view each individual group of three pulses at any instant in time.
- Viewing multiple waveforms is important in evaluating switched mode power supplies since many different test points need to be probed. For example the voltage and current waveforms as shown here.
- By using independent grids on the display, each signal is digitized using full resolution
- The common practice of reducing the waveform to ½ size and then using the offset control to position the traces so that they are not overlapping has the effect of reducing the resolution by as much as one bit. Here we show two voltage waveforms on the same voltage scale except one was acquired at full scale while the other was acquired at ½ scale. The zoom traces compare the resolution of both traces showing the approximately 2x more noise on the signal that was acquired using ½ scale.
- The fast switching speeds encountered in SPMS designs require good probing practice to measure correctly. For example, long ground leads result in high inductance at the probe tip and ringing and overshoot in the time domain signal. In cases such as the one shown here, the ripple voltage is almost 2x as large when measured with a long ground lead.
- Even though switching speeds are generally in the 1 MHz or lower rates, the edges can be very fast and so the bandwidth requirements may be many times higher. In the waveform above the voltage ripple was measured using an active probe which had a bandwidth of greater than 1 GHz. The measured ripple is even lower in this case due to the much lower overshoot of the active probe.
- Both vertical and horizontal resolution are important so sacrificing sampling rate fore more bits is not often a good idea. Short, high amplitude peaks can damage active components so it is important to be able to measure them. For example, a 10 V/ns slew rate increases 1 V between samples when acquired at 10 Gs/s while it covers 5 V between samples when sampled at 2 Gs/s. This is a 5x reduction in resolution but, compared with the resolution of the A/D, sampling rate is the limiting factor.
- To summarize: the best accuracy is achieved by using active probes if possible and keeping leads as short as possible, maximizing sampling rate and bandwidth and using averaging or, where appropriate, high resolution mode.
- Power is measured by first measuring the voltage and current and then computing the power as V*I. It is important to de-skew the current and voltage probes so that the instantaneous value of the power is properly measured. Here we show the result of skew between the current and voltage probes on power in a switching loss measurement. Note that the loss can be either over or under estimated as a result of skew.
- Demonstrate the use of the RT-ZF20 for de-skewing current and voltage probes.