Article submitted by Caelin Gabriel of Shunyata Research.
Understanding power supplies and current delivery.
It is necessary to understand the manner in which power supplies in consumer electronics function. The integrated circuits in consumer electronics require several DC voltages to operate. It is the job of the power supply to convert alternating current (AC) from the wall outlet to direct current (DC) voltages that supply power to the component’s electronic circuits.There are basically two types of power supplies: transformer and transformer-less (switched mode) power supplies. Both use rectifiers that are essentially electronic switches that alternately turn on and off in response to the input AC voltage. It is the rectifiers that convert the AC voltage to a pulsating DC voltage.. This voltage is stored and filtered by the power supply storage capacitors that provide the relatively stable DC voltages to the PCBs and integrated circuits. Unlike a light bulb, fan or simple motor, audio/video power supplies do not pull current in a constant or linear fashion. Rather, they pull current in instantaneous pulses as the rectifiers switch on to fill the storage capacitors. This is as true for low current devices such as CD players and pre-amplifiers as it is for high current amplifiers. The rectifiers turn on and off at the positive and negative voltage crests of the AC waveform. These current pulses have high frequency harmonics up to 50 times the frequency (50, 60 Hz) of the AC power line. This places a great demand upon the AC power circuit and associated connections to deliver current without significant impedance to the current flow.
Placing anything in series with the AC power circuit may restrict, impede or slow instantaneous current delivery. This may also noticeably degrade the performance of the system. This is why most electronics manufacturers discourage the use of power conditioners. They understand that conventional power ‘conditioners’ limit instantaneous current flow and interfere with the performance of their carefully engineered power-supplies.
Isn’t it easy to measure voltage and current with a multimeter?
Conventional multimeters and even dedicated power analyzers and are not designed to detect the burst of current delivered when the rectifiers in a power supply turn on. Rather they ‘average’ the voltage and current over a period of one or more AC power cycles. Further, the probes commonly used to to measure AC current and power have a very low bandwidth and do not accurately detect high frequency impulse current pulses. This is not a problem when measuring the aggregate current and power consumption of a device. However, it does not measure the current and power draw during the actual conduction period of the rectifier. The actual current flow during the conduction period can be 10-50 times the RMS value as measured by a power analyzer.
What is DTCD – Dynamic Transient Current Delivery?
DTCD is method of current analysis that measures instantaneous current in the context of a pulsed current draw. In layman’s terms, it is a way of measuring burst current into typical electronic component power supplies. It enables the measurement of current pulses through a variety of AC power related components, including: power wiring, outlets, distribution panels, terminals, connectors, switches, breakers, power cords and power distributors.
Shunyata Research developed the DTCD Analyzer specifically to accurately measure dynamic transient current pulses. The analyzer has two primary functions. The first provides a power source that simulates AC power grid with an a stable reference voltage and the ability to deliver very high energy current pulses. The second section of the DTCD Analyzer simulates the action of the power supply in a high powered amplifier with integral internal sensors that can detect and analyze the current waveform. It supplies a precision reference voltage to the DUT (device under test) and measures its ability to conduct current during a short gate time (milliseconds). The DTCD Analyzer provides a read-out of the equivalent current (DTCD-I) that the DUT could deliver in a one second time period. It also calculates the equivalent voltage drop (DTCD-Vd) and corresponding impedance (DTCD-Ƶ).
Why is the amperage in the graphs so high?
You may be thinking that your CD player only pulls about one amp of current and your amplifier only draws about 12 amps. So how can a test be valid that shows the power cord pulling hundreds of amps of current?
Remember that power supplies pull current in pulses and this is typically only about 5% to 10% (or less) of the AC duty cycle. During the conduction period, when the cable is actually conducting current, the instantaneous current could be hundreds of amps, but the RMS average is only one to 20 amps, depending upon the device and the load.
Example: If a power supply is drawing 10 amps of current (as measured by a standard current meter); the peak currents would be 10 to 20 times higher or 100-200 amps of instantaneous current during the rectifier’s conduction periods.
It appears from the graph that the commodity power cord has a voltage drop of about 40-50%. How is this possible?
The answer is similar to the answer above. Since the conduction period is short and fast, the cable experiences an instantaneous change in current. The impedance and inductance of the cable resists the change to the current pulse and causes a corresponding instantaneous voltage drop across the cable. Technically there is not a significant RMS voltage drop to the component because the rectifiers will compensate for the instantaneous current reduction by remaining for a longer conduction period. (This effect needs an entire white paper just to explain why this extended conduction period has a direct correlation to sonic performance. It is one of the most significant reasons why some power cords ‘sound’ different.)
Microseconds seems like an unreasonably short period of time to measure current?
Since power supplies pull current in pulses and the pulse duration is typically less than 10% of the duty cycle, the conduction period is typically 200-800 microseconds. The time scale for the graphs is about 50 microseconds from beginning to end. Notice that the slope of the measured waveforms levels out and stabilizes within that 50 microsecond timeframe. Therefore, it is unnecessary to display information beyond the initial 50 microseconds. In other words, the stabilized measured differences would be the same even if we extended the time period beyond that shown.
If the standard power cord slows current delivery, doesn’t it just take a bit longer to fill the storage capacitors?
This is true and explains why the power supply will function within normal average voltage and current requirements. However, that does not mean that there are not audible differences between a cord with better DTCD. A cord with higher instantaneous current delivery will fill the storage capacitors faster. Therefore, the rectifiers are on for a shorter period of time. The longer it takes to fill the storage capacitors means that the peak of the charging waveform has passed while still trying to charge the storage capacitors, thus it is not able to fully charge the storage capacitors. Also, note the voltage drop in the power cable limits the ultimate voltage level that the filter capacitors can be charged to.
A power cable that is capable of 300 to 400 amps of instantaneous charging current will have a much shorter charging time than a cable only able to supply 100 amps. At only 100 amps of Impeded DTCD limits the ability to fully charge the storage capacitors causing a drop in peak voltage. Conversely, a high DTCD reduces the amount of time that the power supply is conducting current and in a low impedance mode. When the rectifiers are on, power line noise is more likely to be transmitted through to the power supply.
Is it possible to measure ‘actual live’ power lines for DTCD?
This is already possible and relatively easy to perform. This test is called the ASCC – Available Short-Circuit Current test. This test is used by electricians in commercial building installations to ensure that a line meets the required current delivery when under load. It tests the electrical integrity of line wiring, splices, connectors, outlets, terminals and breakers in the AC circuit being tested. Typical ASCC test results are in the range of 400-1200 amps of instantaneous current.
It allows you to test your dedicated line to test the level of DTCD available through the power circuit. It also will detect bad circuit breakers, poor junction box connections, bad connections to outlets and undersized wiring in the wall.
These devices are available from several companies that make test equipment and cost around a few hundred dollars.
Watch this video for a demonstration of ASCC and a DTCD comparison of two different power cords.