LS Rect Diode.md
Synchronous Rectification DC-DC Converters

MOSBD (MOSFET with SBD)

<![CDATA[]]>https://mm.digikey.com/Volume0/opasdata/d220001/medias/docus/379/Mosfets...<![CDATA[]]>


wire inductance is problabmatic
there are MOSBD: mosfets with built-in schottky diode
<![CDATA[]]>https://eepower.com/new-industry-products/toshiba-announces-five-new-30v...<![CDATA[]]>


mosbd (aka FETKEYs)


todo do they still exist?
<![CDATA[]]>https://electronics.stackexchange.com/questions/722022/what-happened-to-...<![CDATA[]]>


<![CDATA[]]>https://www.ti.com/lit/wp/snva595a/snva595a.pdf<![CDATA[]]>

choosing:
<![CDATA[]]>https://www.ti.com/lit/an/slyt358/slyt358.pdf?ts=1723233149127<![CDATA[]]>
<![CDATA[]]>https://electronics.stackexchange.com/questions/676063/reverse-recovery-...<![CDATA[]]>

Increasing sync buck efficiency with a diode



A Schottky diode in parallel to the LS switch can increase power conversion efficiency of the buck converter:


A Schottky diode with lower forward voltage than the MOSFETs body diode will decrease losses during dead-time.
Potential decrease or complete elimination of reverse recovery losses if the. Schottky have neglitable turn-on losses


A diode can incorporate additional losses to the system:


For PN-Diodes, during HS turn-on (diode turn-off), the reverse voltage can cause reverse recovery. Schottky diodes do
not suffer from this. (
see <![CDATA[]]>Losses in Power Diodes.<![CDATA[]]>)
parasitic junction capacitance (see below)
parasitic inductances between external diode and mosfet can cause mosfet self turn-on and additional conduction loss (
see <![CDATA[]]>Toshiba Mosfet Product Guide 2009<![CDATA[]]>)


For lowest losses in a sync buck, the diode should then have these characteristics:


Schottky barrier diodes (no PN reverse recovery)
Low forward-voltage to prevent MOSFET body diode turn on (MOSFET reverse recovery)
Lower forward-voltage than MOSFET to reduce conduction loss during dead-time
small lead inductance, SMD


Schottky diodes can have a significant junction capacitance, which looks similar to the reverse recovery effect of the
body diode, but at a smaller
magnitude (<![CDATA[]]>https://ez.analog.com/ez-blogs/b/engineerzone-spotlight/posts/how-a-smal...<![CDATA[]]>)

CSD19506KCS and ST40100C





Here's a V-I-plot of the CSD19506KCS mosfet body diode and ST40100C overlay:



^ Diode forward voltage of CSD19506KCS and ST40100C in an overlay-plot.
At 25°C ST40100C's fwd voltage is lower up to to ~28A.
At 125°C the curves cross at ~15A.

Diode Selection

<![CDATA[]]>https://www.digikey.de/short/382502p8<![CDATA[]]>




  

  

  
Vpeak
  
Iav
  
Ipr
  
Vf_typ @25°15A
  
Vf_max_125
  
Ct_typ(5V)
  





  
ST40100C
  

  
100V
  
2*20A
  

  
0.65
  
0.7
  
845pF *2
  



  
ST10100S
  

  
100V
  
15A
  

  
0.68
  

  

  



  
ST20100S
  

  
100V
  
20A
  

  
0.67
  

  

  



  
ST15100S
  
TO-277B
  
100V
  

  

  
0.68
  

  

  



  
ST3050DJF
  

  

  

  

  

  

  

  



  
MBR30200CT (smc)
  

  
200V
  
2*15A
  
20A*2
  
0.85  (0.9 max)
  
0.75
  
200pF  *2 (400pf max)
  
<![CDATA[]]>pdf<![CDATA[]]>


  
STPS60170C
  

  
170V
  
2*30A
  

  
0.94 (max)
  
0.76
  
1200pF *2
  
<![CDATA[]]>https://www.st.com/resource/en/datasheet/stps60170c.pdf<![CDATA[]]>


  
mcc MBR20200CT
  

  
200V
  
2*
  

  

  

  

  



  
mbr30200ct (diotec)
  

  
200V
  

  

  
0.97
  

  
700pF *2
  
<![CDATA[]]>https://diotec.com/request/datasheet/mbr30100ct.pdf<![CDATA[]]>


  
mbr30150ct (diotec)
  

  

  

  

  
0.85
  

  
700pF *2
  



  
UF160FCT (panjit)
  

  
200V
  

  

  
1.0
  

  
170pF *2
  
trr=50ns       <![CDATA[]]>https://www.panjit.com.tw/upload/datasheet/UF1600FCT_SERIES.pdf<![CDATA[]]>


  
MBRD20200CT (smc)
  

  
200V
  
2*10A
  

  
0.95
  

  

  



  
MBR3040FCT
  

  
200V
  
2*15A?
  

  
0.88
  

  
???
  
<![CDATA[]]>https://www.panjit.com.tw/upload/datasheet/MBR3040FCT_SERIES.pdf<![CDATA[]]>


  
16CTQ150(smc)
  

  
150V
  
2*8A
  

  
0.82
  

  
400pF
  
<![CDATA[]]>https://www.smc-diodes.com/propdf/16CTQ150<![CDATA[]]>(S)(-1)%20N0667%20REV.A.pdf


  
RB228NS150
  

  
150
  
300
  

  
0.82 (0.88 max)
  

  

  



  

  

  

  

  

  

  

  

  





Power Diode Losses

Reverse Recovery Loss

Schottky diodes have almost no reverse recovery.

Junction Capacitance Loss

Junction capacitance can be modeled with a capacitor parallel to the diode:



Charging the capacitor causes power loss, as it current flows during HS turn-on.
The energy stored in the parasitic capacitance is mostly recovered into the coil during LS diode turn-on, however the
inefficient resistive charging across the HS already wastes most of the energy. TODO.

This power loss is proportional ~ V**1.5 and usually only significant for voltages > 100V.

(From Fundamentals of Power Electronics, Robert W. Erickson, Fig 4.52)

Ct depends on Vr. The higher the reverse voltage is, the lower the capacitance:



In the book "Fundamentals of Power Electronics" 4.3.3 (pg 98): C(V) ~ 1/√V
C(V) ~= C0 * √(V0/V) (V0, C0 part constants, pg 81ff)
C(V) ~= C0'/√V      (Eq 4.5)
C0' = C0 * √V0

C0' can be found in the datasheet from a single point of the Ct-chart.

For the MBR30200CT we read:




  
V
  
C
  
C√V




  
2
  
300pF
  
424


  
5
  
200pF
  
447


  
10
  
150pF
  
474


  
25
  
100pF
  
500




Due to reading errors and because the 1/√-dependency is just an approximation, the C√V is not constant.

So let's do a quick scipy.curve_fit:

from scipy.optimize import curvefit
f = lambda v,c0p,e: c0p * (v ** e)      # the C(V) model
xdata = (2,5,10,25)                     # from MBR30200CT datasheet
ydata = (300e-12,200e-12,150e-12,100e-12)
popt, pcov = curvefit(f, xdata, ydata)
c0p, e = popt


c0p = 405e-12 [F√V], e = -0.43



C(v) ~= 405 * v^-0.43 (for the MBR30200CT).

A lazy alternative would be to read the C0' ~= C(~0) from the chart and assume e = -0.5 .

Now we can integrate v*C(v) over v=0..V to find the total energy stored during turn-on off the HS switch:

v*C(v) = v * C0' * v^e = C0' * v^(1+e)


Energy lost during HS turn-on:

W(V) = C0' * V**(2+e) / (2+e)


MBR30200CT: W(V) = 405e-12 * V**1.57 / 1.57



Average power loss:

P(V,fsw) = fsw * C0' * V**(2+e) / (2+e)


MBR30200CT: P(100V,40khz) = 14mW



Using the lazy simplification:

P(V,fsw) = fsw * C(~0V) * V**1.5 / 1.5


MBR30200CT: P(100V,40khz) = 11mW   (25% err)



Conduction

Forward voltage of Si Schottky diodes decreases with rising temperature.

References:


<![CDATA[]]>https://lemuruniovi.com/wp-content/uploads/2021/03/Losses-in-Power-Diode...<![CDATA[]]>
Fundamentals of Power Electronics, Ed. 2001, Robert W. Erickson, Dragan Maksimović, Springer US, ISBN 9780306480485
<![CDATA[]]>https://u.dianyuan.com/bbs/u/41/1148348695.pdf<![CDATA[]]>