December, 2012
www.us-tech.com
Measuring Losses in Switch-Mode Power Supplies Continued from page 51
With older versions of power
sources of potential error. This includes, among other things, elimi- nating any timing skew between voltage and current probes. A helpful accessory is a deskew calibration source. A second key consideration is elimination of DC offset errors by auto zeroing the probes. With potential equipment-relat-
ed foibles addressed, let’s take a clos- er look at what we mean by the loss- es incurred in a switch-mode power
analysis software, measurements of the turn-on, turn-off, and conduction losses required a labor-intensive setup that involved gated parame- ters and cursors. Calculation of the area underneath the voltage and cur- rent curves depended on the place- ment of those parameter gates. Those area calculations are what the oscilloscope used to determine the losses.
Today, there are oscilloscopes
that can be equipped with power analysis software that automates the measurement process. Teledyne LeCroy’s HDO high definition oscillo- scope is one such instrument. A typical screen setup on an
HDO oscilloscope for measurement and analysis of switch-mode power supply losses might entail three grids. The top grid shows the voltage waveform, in which it can be seen that voltages are very low during the conducting portion of a cycle and very high when the transistor is in the off state. A second grid displays the current waveform, where users can see how the current ramps up during the period in which the series inductor is charged. This period cor- responds to the power spike that appears in the third grid, which tracks device power dissipation. The spike is the result of the current for the inductor being dissipated in the channel’s resistance.
Page 53
The improvement in the newer
method lies in its relabeling and sep- arating out of the various parts of the transistor’s switching cycle. Rather than using cursors and gated meas- urements to arrive at turn-on, turn- off, and conduction losses, they are measured independently and then read out independently. No longer does the user have to struggle with setting up parameter gates. It’s a matter of probing the circuit and directly reading out the results. Contact: Teledyne LeCroy, 700
Chestnut Ridge Road, Chestnut Ridge, NY 10977 % 800-553-2769 or 845-425-2000 fax: 845-578-5985 E-mail:
contact.corp@
lecroy.com Web:
http://teledynelecroy.com r
HDP4104 1GHz, high definition oscilloscope.
supply. In such circuits, a switching transistor (usually a field-effect tran- sistor or FET) is switched on and off at a fixed frequency. Each time the transistor switches on to conduct cur- rent, a given amount of energy is transferred to the output. The output voltage is measured internally and that value is passed to a controller that regulates the supply’s duty cycle. If the voltage falls below the desired value, the duty cycle is increased, and if it rises too high, it is decreased, all in the interest of main- taining a steady output-voltage value.
When the output transistor is
conducting current, there is a small voltage drop across the small chan- nel resistance RDS(ON). Meanwhile, the transistor feeds an inductor in series. The current in that inductor cannot change instantaneously when the transistor switches on, but rather ramps up over some period of time. When the transistor turns off, energy stored in the inductor bleeds off into
During the time period in which the transistor is turned off, no current
flows, but high voltages are present across it and very little power is dissipated.
the secondary, which constitutes the mechanism of energy transfer to downstream circuitry. During the time period in which
the transistor is turned off, no cur- rent flows, but high voltages are present across it and very little power is dissipated. While the tran- sistor is conducting, voltage across it is very low, perhaps 1 to 2 V. This is the drop across RDS(ON), known as the IR drop, which, when multiplied by the current, gives us the conduc- tion loss.
Transition Power Losses Power dissipated in the transis-
tor is obviously low when it is in the off state, and is also low while it is conducting. But when it’s transition- ing in state from off to on, and then from on to off, the voltage and cur- rent both change at a finite rate. Power is dissipated by the device and that’s power lost as heat.
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