In general, SynQor's converters have significantly lower output noise and ripple when compared to traditional isolated converters. This is due to SynQor's patented power topology, and excellent design practices. Filtering typically helps reduce the output voltage ripple provided that the filter is designed to attenuate the main frequency components of the ripple. The simplest filter is a differential capacitor across the output of the converters. We typically recommend to customers to empirically try various capacitors at the output of the converter to meet their specific requirements. The reason we recommend that you try different capacitance values empirically is because the effective attenuation that can be provided by a filter capacitor will depend on the output impedance of the converter and load distribution network in addition to the parasitic inductance and resistance of the filter capacitor. In the diagram shown below in Figure 1, Zout is the output impedance of the converter, Ldis and Rdis are the parasitic impedances of the output voltage distribution network and C1, ESR and ESL and the parasitic impedances of the additional filter capacitors.
Figure 1. Converter Output Elements impacting output ripple.
The actual attenuation that can be achieved will be dependent on the layout and interconnect impedances and thus ultimately dependent on the details of the customer’s specific implementation.
However, we can provide some general guidance for managing the output voltage ripple. We generally encourage our customers with critical output voltage ripple requirements to design their first boards with as much filtering as possible. Please make sure that you do not exceed the maximum output capacitance as specified in the datasheet. It is fairly simple to remove (or bypass) filter elements; it is much more difficult to add them if what you have is insufficient.
1. If you are restricted to only using capacitors, then we suggest starting with one of SynQor’s evaluation boards, then you measure output ripple and add capacitors until you are able to meet your system requirements. The available evaluation boards are listed in our selection guide on our web: Evaluation Board Selection Guide. We recommend the following approach for board layout:
a. Use surface mount X7R ceramic capacitors. They have comparatively low ESL/ESR characteristics and superior temperature properties (compared to other ceramic technologies and packages).
b. Position these capacitors close to the pins of the module.
c. Route your output copper so that the output current is forced to flow as close to the terminals of these filter capacitors as is practical.
2. If you have the space, you may want to consider adding an output filter inductor. Note – if you do this, we recommend that you connect the sense leads between the output of the converter and the input to the inductor as shown in Figure 1. If you put the sense leads on the load side of the filter inductor, you are introducing a new pole/zero into the plant and will affect the system stability margins. We have copied a simple spreadsheet example below in Figure 2 to show you how we go about calculating the value of an effective and well damped filter network. Please refer to Figure 2 below for a methodology for designing an effective L/C filter to reduce output ripple of 500mV to 50mV.
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Figure 2. Suggested LC filter to reduce output ripple for NQ60W60HGx40 converter operating at the same input and output voltage of 28 Vdc (as an example).​
3. Finally, measuring output ripple can be challenging. Please refer to the “Output Voltage Ripple Measurements” section below for more information.
Output Voltage Ripple Measurements
It is first important to determine if the excessive noise you are measuring is real and not reflected or common mode noise due to the test setup. How you measure noise is critical to determining whether the level is acceptable or if more filtering is required. SynQor has an application note that addresses this issue in detail: http://www.synqor.com/documents/appnotes/appnt_Vout_Ripple_Measurement.pdf.
We performed comparison tests to measure output noise using three techniques.
I. The first technique used a regular scope probe with a ground wire.
II. The second technique used a BNC connected to the output voltage measurement point (unterminated) on an evaluation board.
III. The third technique used a BNC cable cut at the power supply end with the braid soldered to the output return of the converter and a 50 ohm resistor from the positive lead to the Vout+ of the power converter.
Noise levels ranged from approximately 1 V (using the first technique) to 20 mV (using the third technique). Note how broadly results can vary depending on how the noise is measured.
We recommend, of course, using method number III. But whichever approach you take, it is important to verify that your measurement system is accurate. A simple, but effective test of a measurement system is to take the positive end of the measurement device and the return end of the measurement device, short them together and place them on the output return of the power converter. If you see anything on the scope, then there are problems in your measurement system. Normally, your measurement should yield a zero voltage reading.
An output voltage ripple measurement is only meaningful if the impedance of the load network is well defined. We have specified in our datasheets the actual output capacitors that were used for measuring the datasheet specified output voltage ripple. We could have used different output capacitances, but the parallel combination of capacitors shown in figure 3 that we used for our output ripple measurements represents a ‘typical’ application (based on our experience applying our products in many different types of applications).
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Figure 3. Output impedance used to characterize output voltage ripple for the NQ60W60HG (as an example).
If you still require support, please contact our Technical Support team to obtain help with your particular system.