« Previously: Choosing proper Csnb value for snubber design  
This final paragraph presents a comparison between the method proposed in this paper and those recommended in traditional application notes. As reference, the circuit in Figure 1 will be used to run simulations with Simplis 8.0. Unless otherwise noted, these are the setup conditions:

Vin = 12V, R1 = 10mΩ, L1 = 1.5nH, C1 = 0.5nF, Fsw = 500kHz, VxNOSNB = 23.72V

20170713_EDNA_Maxim-snubber_04_01 (cr) Figure 1: Simplis model used for performance comparison

Three examples will be provided to show the strength of the proposed method in different conditions.

In the first example, the goal is to keep the ringing voltage below 17V. The oscillation reduction required by the snubber is quite strong.

Traditional application notes provide a single set of parameters regardless of the required ringing attenuation. The values, obtained from Eq.(1) and Eq.(2), depend on the parasitic components only (R1, L1, C1). For this reason, these values will not change across the three different examples proposed hereafter.

20170713_EDNA_Maxim-snubber_04_02 (cr)

With this set of values, the Simplis simulation shows a maximum Vx voltage of VxMAX_AN = 20.1V. Even though an attenuation is achieved, the specs are not met.

Let’s see what happens with the method proposed in this paper, which consists of three main steps.

  • Calculate the required gain from Eq.(32). Gsnb = 0.717, i.e. the snubber needs to reduce the Vx peak to 71.7% of the existing Vx peak level.
  • Calculate Csnb with Eq.(39). In this case, Csnb > 1.3nF. Let’s use Csnb = 1.5nF for this example.
  • Finally, knowing Csnb, use Eq.(31) to calculate the optimum Rsnb = 1.82Ω

Simplis simulation using these values show a maximum peak voltage of 16.9V, which meets the requested specs.

The simulation for this first example is shown in Figure 2. While the values provided using the traditional way don’t satisfy the project requirements, the method explained in this article provides just the optimum values to obtain the right attenuation.

20170713_EDNA_Maxim-snubber_04_03 (cr) Figure 2: Simulation of the first two examples

In the second example, the target spec is Vx ≤ 19.8V. Following the same three steps described above, we get Csnb = 0.5nF and Rsnb = 3Ω that, according to simulations, attenuate the ringing value to a maximum of 19.8V (right on spec) compared to the same 20.1V guaranteed by traditional approaches. This example highlights the importance of the proper selection for Rsnb. In both cases, in fact, the value of Csnb is the same (Csnb = 0.5nF). Having a way to calculate the optimum snubber resistor makes a difference between passing or failing the specs using the same Csnb, i.e. the same efficiency drop.

Finally, in the last example, the required reduction is even smaller with Vx ≤ 21V. Again, the three steps mentioned previously lead to Csnb = 0.3nF and Rsnb = 4.16Ω with a maximum ringing value of 21V. In this case, both methods lead to solutions that meet the specs. Once again, though, the new method provides a better solution guaranteeing the right voltage level with a lower Csnb, which, in turn, means lower loss and better overall efficiency.

All of this data is summarized in Table 1.

20170713_EDNA_Maxim-snubber_04_04 (cr) Table 1: Summary of example 1. Snubber values recommended by online app notes don’t meet the design target. Those obtained with the approach presented in this paper do.

Conclusions

This paper presented a new method to choose the snubber values Rsnb and Csnb. Rsnb is calculated using Eq.(31), which was derived through rigorous frequency-domain analysis. Csnb, instead, is determined using Eq.(39), which was obtained by exploiting numerical analysis.

The steps to determine the optimum values of the snubber are summarised here:

  • Determine how much gain the snubber must introduce using Eq.(32).

20170713_EDNA_Maxim-snubber_04_05 (cr)

  • Measure the parasitic components

20170713_EDNA_Maxim-snubber_04_06 (cr)

  • Calculate the value of Csnb with Eq.(39). Use Eq.(31) to determine the optimum value for Rsnb.

20170713_EDNA_Maxim-snubber_04_07 (cr)

The last paragraph presents a result comparison between the traditional method and the one proposed in this paper to determine the snubber values. It shows how the latter provides a better tuned circuit that always leads to a solution that meets the design specification with overall lower power dissipation.

First published by EDN.

 
« Previously: Choosing proper Csnb value for snubber design